Method and apparatus for video decoding that defines a search range for a reference block indicated by a block vector

ABSTRACT

A method and an apparatus for video decoding are disclosed. The apparatus decodes prediction information of a current block from a coded video bitstream. The prediction information indicates an intra block copy mode. The current block is one of a plurality of coding blocks in a current region of a current coding tree block (CTB) in a current picture. The apparatus determines whether the current block is to be reconstructed first in the current region. When the current block is to be reconstructed first in the current region, the apparatus determines a block vector for the current block where a reference block indicated by the block vector is in a search range in the current picture that excludes a collocated region in a previously reconstructed CTB. A position of the collocated region in the previously reconstructed CTB has a same relative position as the current region in the current CTB.

INCORPORATION BY REFERENCE

This present disclosure claims the benefit of priority to U.S.Provisional Application No. 62/816,125, “Search Range Adjustment forintra Picture Block Compensation” filed on Mar. 9, 2019, and U.S.Provisional Application No. 62/735,002, “Reference Search RangeOptimization for intra Picture Block Compensation” filed on Sep. 21,2018, which are incorporated by references herein in their entirety.

TECHNICAL FIELD

The present disclosure describes embodiments generally related to videocoding.

BACKGROUND

The background description provided herein is for the purpose ofgenerally presenting the context of the disclosure. Work of thepresently named inventors, to the extent the work is described in thisbackground section, as well as aspects of the description that may nototherwise qualify as prior art at the time of filing, are neitherexpressly nor impliedly admitted as prior art against the presentdisclosure.

Video coding and decoding can be performed using inter-pictureprediction with motion compensation. Uncompressed digital video caninclude a series of pictures, each picture having a spatial dimensionof, for example, 1920×1080 luminance samples and associated chrominancesamples. The series of pictures can have a fixed or variable picturerate (informally also known as frame rate), of, for example 60 picturesper second or 60 Hz. Uncompressed video has significant bitraterequirements. For example, 1080p60 4:2:0 video at 8 bit per sample(1920×1080 luminance sample resolution at 60 Hz frame rate) requiresclose to 1.5 Gbit/s bandwidth. An hour of such video requires more than600 GBytes of storage space.

One purpose of video coding and decoding can be the reduction ofredundancy in the input video signal, through compression. Compressioncan help reduce the aforementioned bandwidth or storage spacerequirements, in some cases by two orders of magnitude or more. Bothlossless and lossy compression, as well as a combination thereof can beemployed. Lossless compression refers to techniques where an exact copyof the original signal can be reconstructed from the compressed originalsignal. When using lossy compression, the reconstructed signal may notbe identical to the original signal, but the distortion between originaland reconstructed signals is small enough to make the reconstructedsignal useful for the intended application. In the case of video, lossycompression is widely employed. The amount of distortion tolerateddepends on the application; for example, users of certain consumerstreaming applications may tolerate higher distortion than users oftelevision distribution applications. The compression ratio achievablecan reflect that: higher allowable/tolerable distortion can yield highercompression ratios.

A video encoder and decoder can utilize techniques from several broadcategories, including, for example, motion compensation, transform,quantization, and entropy coding.

Video codec technologies can include techniques known as intra coding.In intra coding, sample values are represented without reference tosamples or other data from previously reconstructed reference pictures.In some video codecs, the picture is spatially subdivided into blocks ofsamples. When all blocks of samples are coded in intra mode, thatpicture can be an intra picture. Intra pictures and their derivationssuch as independent decoder refresh pictures, can be used to reset thedecoder state and can, therefore, be used as the first picture in acoded video bitstream and a video session, or as a still image. Thesamples of an intra block can be exposed to a transform, and thetransform coefficients can be quantized before entropy coding. Intraprediction can be a technique that minimizes sample values in thepre-transform domain. In some cases, the smaller the DC value after atransform is, and the smaller the AC coefficients are, the fewer thebits that are required at a given quantization step size to representthe block after entropy coding.

Traditional intra coding such as known from, for example MPEG-2generation coding technologies, does not use intra prediction. However,some newer video compression technologies include techniques thatattempt, from, for example, surrounding sample data and/or metadataobtained during the encoding/decoding of spatially neighboring, andpreceding in decoding order, blocks of data. Such techniques arehenceforth called “intra prediction” techniques. Note that in at leastsome cases, intra prediction is only using reference data from thecurrent picture under reconstruction and not from reference pictures.

There can be many different forms of intra prediction. When more thanone of such techniques can be used in a given video coding technology,the technique in use can be coded in an intra prediction mode. Incertain cases, modes can have submodes and/or parameters, and those canbe coded individually or included in the mode codeword. Which codewordto use for a given mode/submode/parameter combination can have an impactin the coding efficiency gain through intra prediction, and so can theentropy coding technology used to translate the codewords into abitstream.

A certain mode of intra prediction was introduced with H.264, refined inH.265, and further refined in newer coding technologies such as jointexploration model (JEM), versatile video coding (VVC), and benchmark set(BMS). A predictor block can be formed using neighboring sample valuesbelonging to already available samples. Sample values of neighboringsamples are copied into the predictor block according to a direction. Areference to the direction in use can be coded in the bitstream or mayitself be predicted.

Referring to FIG. 1, depicted in the lower right is a subset of ninepredictor directions known from H.265's 33 possible predictor directions(corresponding to the 33 angular modes of the 35 intra modes). The pointwhere the arrows converge (101) represents the sample being predicted.The arrows represent the direction from which the sample is beingpredicted. For example, arrow (102) indicates that sample (101) ispredicted from a sample or samples to the upper right, at a 45 degreeangle from the horizontal. Similarly, arrow (103) indicates that sample(101) is predicted from a sample or samples to the lower left of sample(101), in a 22.5 degree angle from the horizontal.

Still referring to FIG. 1, on the top left there is depicted a squareblock (104) of 4×4 samples (indicated by a dashed, boldface line). Thesquare block (104) includes 16 samples, each labelled with an “S”, itsposition in the Y dimension (e.g., row index) and its position in the Xdimension (e.g., column index). For example, sample S21 is the secondsample in the Y dimension (from the top) and the first (from the left)sample in the X dimension. Similarly, sample S44 is the fourth sample inblock (104) in both the Y and X dimensions. As the block is 4×4 samplesin size, S44 is at the bottom right. Further shown are reference samplesthat follow a similar numbering scheme. A reference sample is labelledwith an R, its Y position (e.g., row index) and X position (columnindex) relative to block (104). In both H.264 and H.265, predictionsamples neighbor the block under reconstruction; therefore no negativevalues need to be used.

Intra picture prediction can work by copying reference sample valuesfrom the neighboring samples as appropriated by the signaled predictiondirection. For example, assume the coded video bitstream includessignaling that, for this block, indicates a prediction directionconsistent with arrow (102)—that is, samples are predicted from aprediction sample or samples to the upper right, at a 45 degree anglefrom the horizontal. In that case, samples S41, S32, S23, and S14 arepredicted from the same reference sample R05. Sample S44 is thenpredicted from reference sample R08.

In certain cases, the values of multiple reference samples may becombined, for example through interpolation, in order to calculate areference sample; especially when the directions are not evenlydivisible by 45 degrees.

The number of possible directions has increased as video codingtechnology has developed. In H.264 (year 2003), nine different directioncould be represented. That increased to 33 in H.265 (year 2013), andJEM/VVC/BMS, at the time of disclosure, can support up to 65 directions.Experiments have been conducted to identify the most likely directions,and certain techniques in the entropy coding are used to represent thoselikely directions in a small number of bits, accepting a certain penaltyfor less likely directions. Further, the directions themselves cansometimes be predicted from neighboring directions used in neighboring,already decoded, blocks.

FIG. 2 shows a schematic (201) that depicts 65 intra predictiondirections according to TEM to illustrate the increasing number ofprediction directions over time.

The mapping of intra prediction directions bits in the coded videobitstream that represent the direction can be different from videocoding technology to video coding technology; and can range, forexample, from simple direct mappings of prediction direction to intraprediction mode, to codewords, to complex adaptive schemes involvingmost probable modes, and similar techniques. In all cases, however,there can be certain directions that are statistically less likely tooccur in video content than certain other directions. As the goal ofvideo compression is the reduction of redundancy, those less likelydirections will, in a well working video coding technology, berepresented by a larger number of bits than more likely directions.

SUMMARY

Aspects of the disclosure provide methods and apparatuses for videoencoding/decoding. In some examples, an apparatus for video decodingincludes processing circuitry. The processing circuitry decodesprediction information of a current block from a coded video bitstreamwhere the prediction information is indicative of an intra block copymode and the current block is one of a plurality of coding blocks in acurrent region of a current coding tree block (CTB) in a currentpicture. The processing circuitry determines whether the current blockis to be reconstructed first in the current region. When the currentblock is to be reconstructed first in the current region, the processingcircuitry determines a block vector for the current block. A referenceblock indicated by the block vector is in a search range that excludes acollocated region in a previously reconstructed CTB and a position ofthe collocated region in the previously reconstructed CTB has a samerelative position as the current region in the current CTB. The searchrange is in the current picture. The processing circuitry reconstructsat least one sample of the current block according the block vector. Thesearch range can include coding blocks that are reconstructed after thecollocated region and before the current block.

In an embodiment, a size of the current CTB is equal to a referencememory size, the previously reconstructed CTB is a left neighbor of thecurrent CTB, the position of the collocated region is offset by a widthof the current CTB from a position of the current region, and the codingblocks in the search range are in at least one of: the current CTB andthe previously reconstructed CTB.

In an example, the size of the current CTB and the previouslyreconstructed CTB is 128 by 128 samples, the current CTB includes 4regions of 64 by 64 samples, the previously reconstructed CTB includes 4regions of 64 by 64 samples, the position of the collocated region isoffset by 128 samples from the position of the current region, thecurrent region being one of the 4 regions in the current CTB and thecollocated region being one of the 4 regions in the previouslyreconstructed CTB. The 4 regions in the current CTB can include a topleft region, a top right region, a bottom left region, and a bottomright region. The 4 regions in the previously reconstructed CTB caninclude a top left region, a top right region, a bottom left region, anda bottom right region. When the current region is the top left region ofthe current CTB, the collocated region is the top left region of thepreviously reconstructed CTB and the search region excludes the top leftregion of the previously reconstructed CTB. When the current region isthe top right region of the current CTB, the collocated region is thetop right region of the previously reconstructed CTB and the searchregion excludes the top left region and the top right region of thepreviously reconstructed CTB. When the current region is the bottom leftregion of the current CTB, the collocated region is the bottom leftregion of the previously reconstructed CTB and the search regionexcludes the top left region, the top right region, and the bottom leftregion of the previously reconstructed CTB. When the current region isthe bottom right region of the current CTB, the collocated region is thebottom right region of the previously reconstructed CTB and the searchregion excludes the previously reconstructed CTB.

In an example, the current CTB includes 4 regions having a same size andshape, the previously reconstructed CTB includes 4 regions having thesame size and the shape, the current region is one of the 4 regions inthe current CTB, and the collocated region is one of the 4 regions inthe previously reconstructed CTB.

In an embodiment, a size of the current CTB is less than a referencememory size, the position of the collocated region is offset by multiplewidths of the current CTB from a position of the current region, and thecoding blocks in the search range are in at least one of: the currentCTB, the previously reconstructed CTB, and one or more reconstructedCTBs between the current CTB and the previously reconstructed CTB. In anexample, the size of the current CTB is 64×64 samples, the referencememory size is 128×128 samples, the current CTB includes 4 regions of32×32 samples, the previously reconstructed CTB includes 4 regions of32×32 samples, the position of the collocated region is offset by 256samples from the position of the current region. In an example, thecoding blocks in the search range are in at least one of: the currentCTB and the one or more reconstructed CTBs between the current CTB andthe previously reconstructed CTB. In an example, the search rangeexcludes the previously, reconstructed CTB that is offset by N widths ofthe current CTB from the current CTB where N is a ratio of the referencememory size over the size of the current CTB.

Aspects of the disclosure also provide a non-transitorycomputer-readable medium storing instructions which when executed by acomputer for video decoding cause the computer to perform the method forvideo decoding.

BRIEF DESCRIPTION OF THE DRAWINGS

Further features, the nature, and various advantages of the disclosedsubject matter will be more apparent from the following detaileddescription and the accompanying drawings in which:

FIG. 1 is a schematic illustration of an exemplary subset of intraprediction modes.

FIG. 2 is an illustration of exemplary intra prediction directions.

FIG. 3 is a schematic illustration of a simplified block diagram of acommunication system (300) in accordance with an embodiment.

FIG. 4 is a schematic illustration of a simplified block diagram of acommunication system (400) in accordance with an embodiment.

FIG. 5 is a schematic illustration of a simplified block diagram of adecoder in accordance with an embodiment.

FIG. 6 is a schematic illustration of a simplified block diagram of anencoder in accordance with an embodiment.

FIG. 7 shows a block diagram of an encoder in accordance with anotherembodiment.

FIG. 8 shows a block diagram of a decoder in accordance with anotherembodiment.

FIG. 9 shows an example of intra block copy according to an embodimentof the disclosure.

FIG. 10 shows an example of intra block copy according to an embodimentof the disclosure.

FIG. 11 shows an example of intra block copy according to an embodimentof the disclosure.

FIGS. 12A-12D show examples of intra block copy according to anembodiment of the disclosure.

FIG. 13 shows an example of intra block copy having a search range thatis larger than a CTB size according to an embodiment of the disclosure.

FIG. 14 shows a flow chart outlining a process (1400) according to anembodiment of the disclosure.

FIG. 15 is a schematic illustration of a computer system in accordancewith an embodiment.

DETAILED DESCRIPTION OF EMBODIMENTS

FIG. 3 illustrates a simplified block diagram of a communication system(300) according to an embodiment of the present disclosure. Thecommunication system (300) includes a plurality of terminal devices thatcan communicate with each other, via, for example, a network (350). Forexample, the communication system (300) includes a first pair ofterminal devices (310) and (320) interconnected via the network (350).In the FIG. 3 example, the first pair of terminal devices (310) and(320) performs unidirectional transmission of data. For example, theterminal device (310) may code video data (e.g., a stream of videopictures that are captured by the terminal device (310)) fortransmission to the other terminal device (320) via the network (350).The encoded video data can be transmitted in the form of one or morecoded video bitstreams. The terminal device (320) may receive the codedvideo data from the network (350), decode the coded video data torecover the video pictures and display video pictures according to therecovered video data. Unidirectional data transmission may be common inmedia serving applications and the like.

In another example, the communication system (300) includes a secondpair of terminal devices (330) and (340) that performs bidirectionaltransmission of coded video data that may occur, for example, duringvideoconferencing. For bidirectional transmission of data, in anexample, each terminal device of the terminal devices (330) and (340)may code video data (e.g., a stream of video pictures that are capturedby the terminal device) for transmission to the other terminal device ofthe terminal devices (330) and (340) via the network (350). Eachterminal device of the terminal devices (330) and (340) also may receivethe coded video data transmitted by the other terminal device of theterminal devices (330) and (340), and may decode the coded video data torecover the video pictures and may display video pictures at anaccessible display device according to the recovered video data.

In the FIG. 3 example, the terminal devices (310), (320), (330) and(340) may be illustrated as servers, personal computers and smart phonesbut the principles of the present disclosure may be not so limited.Embodiments of the present disclosure find application with laptopcomputers, tablet computers, media players and/or dedicated videoconferencing equipment. The network (350) represents any number ofnetworks that convey coded video data among the terminal devices (310),(320), (330) and (340), including for example wireline (wired) and/orwireless communication networks. The communication network (350) mayexchange data in circuit-switched and/or packet-switched channels.Representative networks include telecommunications networks, local areanetworks, wide area networks and/or the Internet. For the purposes ofthe present discussion, the architecture and topology of the network(350) may be immaterial to the operation of the present disclosureunless explained herein below.

FIG. 4 illustrates, as an example for an application for the disclosedsubject matter, the placement of a video encoder and a video decoder ina streaming environment. The disclosed subject matter can be equallyapplicable to other video enabled applications, including, for example,video conferencing, digital TV, storing of compressed video on digitalmedia including CD, DVD, memory stick and the like, and so on.

A streaming system may include a capture subsystem (413), that caninclude a video source (401), for example a digital camera, creating forexample a stream of video pictures (402) that are uncompressed. In anexample, the stream of video pictures (402) includes samples that aretaken by the digital camera. The stream of video pictures (402),depicted as a bold line to emphasize a high data volume when compared toencoded video data (104) (or coded video bitstreams), can be processedby an electronic device (420) that includes a video encoder (403)coupled to the video source (401). The video encoder (403) can includehardware, software, or a combination thereof to enable or implementaspects of the disclosed subject matter as described in more detailbelow. The encoded video data (404) (or encoded video bitstream (404)),depicted as a thin line to emphasize the lower data volume when comparedto the stream of video pictures (402), can be stored on a streamingserver (405) for future use. One or more streaming client subsystems,such as client subsystems (406) and (408) in FIG. 4 can access thestreaming server (405) to retrieve copies (407) and (409) of the encodedvideo data (404). A client subsystem (406) can include a video decoder(410), for example, in an electronic device (430). The video decoder(410) decodes the incoming copy (407) of the encoded video data andcreates an outgoing stream of video pictures (411) that can be renderedon a display (412) (e.g., display screen) or other rendering device (notdepicted). In some streaming systems, the encoded video data (404),(407), and (409) (e.g., video bitstreams) can be encoded according tocertain video coding/compression standards. Examples of those standardsinclude ITU-T Recommendation 1-1.265. In an example, a video codingstandard under development is informally known as Versatile Video Coding(VVC). The disclosed subject matter may be used in the context of VVC.

It is noted that the electronic devices (420) and (430) can includeother components (not shown). For example, the electronic device (420)can include a video decoder (not shown) and the electronic device (430)can include a video encoder (not shown) as well.

FIG. 5 shows a block diagram of a video decoder (510) according to anembodiment of the present disclosure. The video decoder (510) can beincluded in an electronic device (530). The electronic device (530) caninclude a receiver (531) (e.g., receiving circuitry). The video decoder(510) can be used in the place of the video decoder (410) in the FIG. 4example.

The receiver (531) may receive one or more coded video sequences to bedecoded by video decoder (510); in the same or another embodiment, onecoded video sequence at a time, where the decoding of each coded videosequence is independent from other coded video sequences. The codedvideo sequence may be received from a channel (501), which may be ahardware/software link to a storage device which stores the encodedvideo data. The receiver (531) may receive the encoded video data withother data, for example, coded audio data and/or ancillary data streams,that may be forwarded to their respective using entities (not depicted).The receiver (531) may separate the coded video sequence from the otherdata. To combat network jitter, a buffer memory (515) may be coupled inbetween the receiver (531) and an entropy decoder/parser (520) (“parser(520)” henceforth). In certain applications, the buffer memory (515) ispart of the video decoder (510). In others, it can be outside of thevideo decoder (510) (not depicted). In still others, there can be abuffer memory (not depicted) outside of the video decoder (510), forexample to combat network jitter, and in addition another buffer memory(515) inside the video decoder (510), for example to handle playouttiming. When the receiver (531) is receiving data from a store/forwarddevice of sufficient bandwidth and controllability, or from anisosynchronous network, the buffer memory (515) may not be needed, orcan be small. For use on best effort packet networks such as theInternet, the buffer memory (515) may be required, can be comparativelylarge and can be advantageously of adaptive size, and may at leastpartially be implemented in an operating system or similar elements (notdepicted) outside of the video decoder (510).

The video decoder (510) may include the parser (520) to reconstructsymbols (521) from the coded video sequence. Categories of those symbolsinclude information used to manage operation of the video decoder (510),and potentially information to control a rendering device such as arender device (512) (e.g., a display screen) that is not an integralpart of the electronic device (530) but can be coupled to the electronicdevice (530), as was shown in FIG. 5. The control information for therendering device(s) may be in the form of Supplemental EnhancementInformation (SEI messages) or Video Usability Information (VIII)parameter set fragments (not depicted), The parser (520) mayparse/entropy-decode the coded video sequence that is received. Thecoding of the coded video sequence can be in accordance with a videocoding technology or standard, and can follow various principles,including variable length coding, Huffman coding, arithmetic coding withor without context sensitivity, and so forth. The parser (520) mayextract from the coded video sequence, a set of subgroup parameters forat least one of the subgroups of pixels in the video decoder, based uponat least one parameter corresponding to the group. Subgroups can includeGroups of Pictures (GOPs), pictures, tiles, slices, macroblocks, CodingUnits (CUs), blocks, Transform Units (TUs), Prediction Units (PUs) andso forth. The parser (520) may also extract from the coded videosequence information such as transform coefficients, quantizer parametervalues, motion vectors, and so forth.

The parser (520) may perform an entropy decoding/parsing operation onthe video sequence received from the buffer memory (515), so as tocreate symbols (521).

Reconstruction of the symbols (521) can involve multiple different unitsdepending on the type of the coded video picture or parts thereof (suchas: inter and intra picture, inter and intra block), and other factors.Which units are involved, and how, can be controlled by the subgroupcontrol information that was parsed from the coded video sequence by theparser (520). The flow of such subgroup control information between theparser (520) and the multiple units below is not depicted for clarity.

Beyond the functional blocks already mentioned, the video decoder (510)can be conceptually subdivided into a number of functional units asdescribed below. In a practical implementation operating undercommercial constraints, many of these units interact closely with eachother and can, at least partly, be integrated into each other. However,for the purpose of describing the disclosed subject matter, theconceptual subdivision into the functional units below is appropriate.

A first unit is the scaler inverse transform unit (551). Thescaler/inverse transform unit (551) receives a quantized transformcoefficient as well as control information, including which transform touse, block size, quantization factor, quantization scaling matrices,etc. as symbol(s) (521) from the parser (520). The scaler/inversetransform unit (551) can output blocks comprising sample values, thatcan be input into aggregator (555).

In some cases, the output samples of the scaler inverse transform (551)can pertain to an intra coded block; that is: a block that is not usingpredictive information from previously reconstructed pictures, but canuse predictive information from previously reconstructed parts of thecurrent picture. Such predictive information can be provided by an intrapicture prediction unit (552). In some cases, the intra pictureprediction unit (552) generates a block of the same size and shape ofthe block under reconstruction, using surrounding already reconstructedinformation fetched from the current picture buffer (558). The currentpicture buffer (558) buffers, for example, partly reconstructed currentpicture and/or fully reconstructed current picture. The aggregator(555), in some cases, adds, on a per sample basis, the predictioninformation the intra prediction unit (552) has generated to the outputsample information as provided by the scaler inverse transform unit(551).

In other cases, the output samples of the scaler/inverse transform unit(551) can pertain to an inter coded, and potentially motion compensatedblock. In such a case, a motion compensation prediction unit (553) canaccess reference picture memory (557) to fetch samples used forprediction. After motion compensating the fetched samples in accordancewith the symbols (521) pertaining to the block, these samples can beadded by the aggregator (555) to the output of the scaler/inversetransform unit (551) (in this case called the residual samples orresidual signal) so as to generate output sample information. Theaddresses within the reference picture memory (557) from where themotion compensation prediction unit (553) fetches prediction samples canbe controlled by motion vectors, available to the motion compensationprediction unit (553) in the form of symbols (521) that can have, forexample X, Y, and reference picture components. Motion compensation alsocan include interpolation of sample values as fetched from the referencepicture memory (557) when sub-sample exact motion vectors are in use,motion vector prediction mechanisms, and so forth.

The output samples of the aggregator (555) can be subject to variousloop filtering techniques in the loop filter unit (556). Videocompression technologies can include in-loop filter technologies thatare controlled by parameters included in the coded video sequence (alsoreferred to as coded video bitstream) and made available to the loopfilter unit (556) as symbols (521) from the parser (520), but can alsobe responsive to meta-information obtained during the decoding ofprevious (in decoding order) parts of the coded picture or coded videosequence, as well as responsive to previously reconstructed andloop-filtered sample values.

The output of the loop filter unit (556) can be a sample stream that canbe output to the render device (512) as well as stored in the referencepicture memory (557) for use in future inter-picture prediction.

Certain coded pictures, once fully reconstructed, can be used asreference pictures for future prediction. For example, once a codedpicture corresponding to a current picture is fully reconstructed andthe coded picture has been identified as a reference picture (by, forexample, the parser (520)), the current picture buffer (558) can becomea part of the reference picture memory (557), and a fresh currentpicture buffer can be reallocated before commencing the reconstructionof the following coded picture.

The video decoder (510) may perform decoding operations according to apredetermined video compression technology in a standard, such as ITU-TRec. H-1.265. The coded video sequence may conform to a syntax specifiedby the video compression technology or standard being used, in the sensethat the coded video sequence adheres to both the syntax of the videocompression technology or standard and the profiles as documented in thevideo compression technology or standard. Specifically, a profile canselect certain tools as the only tools available for use under thatprofile from all the tools available in the video compression technologyor standard. Also necessary for compliance can be that the complexity ofthe coded video sequence is within bounds as defined by the level of thevideo compression technology or standard. In some cases, levels restrictthe maximum picture size, maximum frame rate, maximum reconstructionsample rate (measured in, for example megasamples per second), maximumreference picture size, and so on. Limits set by levels can, in somecases, be further restricted through Hypothetical Reference Decoder(HRD) specifications and metadata for HRD buffer management signaled inthe coded video sequence.

In an embodiment, the receiver (531) may receive additional (redundant)data with the encoded video. The additional data may be included as partof the coded video sequence(s). The additional data may be used by thevideo decoder (510) to properly decode the data and/or to moreaccurately reconstruct the original video data. Additional data can bein the form of, for example, temporal, spatial, or signal noise ratio(SNR) enhancement layers, redundant slices, redundant pictures, forwarderror correction codes, and so on.

FIG. 6 shows a block diagram of a video encoder (603) according to anembodiment of the present disclosure. The video encoder (603) isincluded in an electronic device (620). The electronic device (620)includes a transmitter (640) (e.g., transmitting circuitry). The videoencoder (603) can be used in the place of the video encoder (403) in theFIG. 4 example.

The video encoder (603) may receive video samples from a video source(601) (that is not part of the electronic device (620) in the FIG. 6example) that may capture video image(s) to be coded by the videoencoder (603). In another example, the video source (601) is a part ofthe electronic device (620).

The video source (601) may provide the source video sequence to be codedby the video encoder (603) in the form of a digital video sample streamthat can be of any suitable bit depth (for example: 8 bit, 10 bit, 12bit, . . . ), any colorspace (for example, BT.601 Y CrCB, RGB, . . . ),and any suitable sampling structure (for example Y CrCb 4:2:0, Y CrCb4:4:4). In a media serving system, the video source (601) may be astorage device storing previously prepared video. In a videoconferencingsystem, the video source (601) may be a camera that captures local imageinformation as a video sequence. Video data may be provided as aplurality of individual pictures that impart motion when viewed insequence. The pictures themselves may be organized as a spatial array ofpixels, wherein each pixel can comprise one or more samples depending onthe sampling structure, color space, etc. in use. A person skilled inthe art can readily understand the relationship between pixels andsamples. The description below focuses on samples.

According to an embodiment, the video encoder (603) may code andcompress the pictures of the source video sequence into a coded videosequence (643) in real time or under any other time constraints asrequired by the application. Enforcing appropriate coding speed is onefunction of a controller (650). In some embodiments, the controller(650) controls other functional units as described below and isfunctionally coupled to the other functional units. The coupling is notdepicted for clarity. Parameters set by the controller (650) can includerate control related parameters (picture skip, quantizer, lambda valueof rate-distortion optimization techniques, . . . ), picture size, groupof pictures (GOP) layout, maximum motion vector search range, and soforth. The controller (650) can be configured to have other suitablefunctions that pertain to the video encoder (603) optimized for acertain system design.

In some embodiments, the video encoder (603) is configured to operate ina coding loop. As an oversimplified description, in an example, thecoding loop can include a source coder (630) (e.g., responsible forcreating symbols, such as a symbol stream, based on an input picture tobe coded, and a reference picture(s)), and a (local) decoder (633)embedded in the video encoder (603). The decoder (633) reconstructs thesymbols to create the sample data in a similar manner as a (remote)decoder also would create (as any compression between symbols and codedvideo bitstream is lossless in the video compression technologiesconsidered in the disclosed subject matter). The reconstructed samplestream (sample data) is input to the reference picture memory (634). Asthe decoding of a symbol stream leads to bit-exact results independentof decoder location (local or remote), the content in the referencepicture memory (634) is also bit exact between the local encoder andremote encoder. In other words, the prediction part of an encoder “sees”as reference picture samples exactly the same sample values as a decoderwould “see” when using prediction during decoding. This fundamentalprinciple of reference picture synchronicity (and resulting drift, ifsynchronicity cannot be maintained, for example because of channelerrors) is used in some related arts as well.

The operation of the “local” decoder (633) can be the same as of a“remote” decoder, such as the video decoder (510), which has alreadybeen described in detail above in conjunction with FIG. 5. Brieflyreferring also to FIG. 5, however, as symbols are available andencoding/decoding of symbols to a coded video sequence by an entropycoder (645) and the parser (520) can be lossless, the entropy decodingparts of the video decoder (510), including the buffer memory (515), andparser (520) may not be fully implemented in the local decoder (633).

An observation that can be made at this point is that any decodertechnology except the parsing/lentropy decoding that is present in adecoder also necessarily needs to be present, in substantially identicalfunctional form, in a corresponding encoder. For this reason, thedisclosed subject matter focuses on decoder operation. The descriptionof encoder technologies can be abbreviated as they are the inverse ofthe comprehensively described decoder technologies. Only in certainareas a more detail description is required and provided below.

During operation, in some examples, the source coder (630) may performmotion compensated predictive coding, which codes an input picturepredictively with reference to one or more previously-coded picture fromthe video sequence that were designated as “reference pictures”. In thismanner, the coding engine (632) codes differences between pixel blocksof an input picture and pixel blocks of reference picture(s) that may beselected as prediction reference(s) to the input picture.

The local video decoder (633) may decode coded video data of picturesthat may be designated as reference pictures, based on symbols createdby the source coder (630). Operations of the coding engine (632) mayadvantageously be lossy processes. When the coded video data may bedecoded at a video decoder (not shown in FIG. 6), the reconstructedvideo sequence typically may be a replica of the source video sequencewith some errors. The local video decoder (633) replicates decodingprocesses that may be performed by the video decoder on referencepictures and may cause reconstructed reference pictures to be stored inthe reference picture cache (634). In this manner, the video encoder(603) may store copies of reconstructed reference pictures locally thathave common content as the reconstructed reference pictures that will beobtained by a far-end video decoder (absent transmission errors).

The predictor (635) may perform prediction searches for the codingengine (632). That is, for a new picture to be coded, the predictor(635) may search the reference picture memory (634) for sample data (ascandidate reference pixel blocks) or certain metadata such as referencepicture motion vectors, block shapes, and so on, that may serve as anappropriate prediction reference for the new pictures. The predictor(635) may operate on a sample block-by-pixel block basis to findappropriate prediction references. In some cases, as determined bysearch results obtained by the predictor (635), an input picture mayhave prediction references drawn from multiple reference pictures storedin the reference picture memory (634).

The controller (650) may manage coding operations of the source coder(630), including, for example, setting of parameters and subgroupparameters used for encoding the video data.

Output of all aforementioned functional units may be subjected toentropy coding in the entropy coder (645). The entropy coder (645)translates the symbols as generated by the various functional units intoa coded video sequence, by lossless compressing the symbols according totechnologies such as Huffman coding, variable length coding, arithmeticcoding, and so forth.

The transmitter (640) may buffer the coded video sequence(s) as createdby the entropy coder (645) to prepare for transmission via acommunication channel (660), which may be a hardware/software link to astorage device which would store the encoded video data. The transmitter(640) may merge coded video data from the video coder (603) with otherdata to be transmitted, for example, coded audio data and/or ancillarydata streams (sources not shown).

The controller (650) may manage operation of the video encoder (603).During coding, the controller (650) may assign to each coded picture acertain coded picture type, which may affect the coding techniques thatmay be applied to the respective picture. For example, pictures oftenmay be assigned as one of the following picture types:

An Intra Picture (I picture) may be one that may be coded and decodedwithout using any other picture in the sequence as a source ofprediction. Some video codecs allow for different types of intrapictures, including, for example Independent Decoder Refresh (“IDR”)Pictures. A person skilled in the art is aware of those variants of Ipictures and their respective applications and features.

A predictive picture (P picture) may be one that may be coded anddecoded using intra prediction or inter prediction using at most onemotion vector and reference index to predict the sample values of eachblock.

A bi-directionally predictive picture (B Picture) may be one that may becoded and decoded using intra prediction or inter prediction using atmost two motion vectors and reference indices to predict the samplevalues of each block. Similarly, multiple-predictive pictures can usemore than two reference pictures and associated metadata for thereconstruction of a single block.

Source pictures commonly may be subdivided spatially into a plurality ofsample blocks (for example, blocks of 4×4, 8×8, 4×8, or 16×16 sampleseach) and coded on a block-by-block basis. Blocks may be codedpredictively with reference to other (already coded) blocks asdetermined by the coding assignment applied to the blocks' respectivepictures. For example, blocks of I pictures may be codednon-predictively or they may be coded predictively with reference toalready coded blocks of the same picture (spatial prediction or intraprediction). Pixel blocks of P pictures may be coded predictively, viaspatial prediction or via temporal prediction with reference to onepreviously coded reference picture. Blocks of B pictures may be codedpredictively, via spatial prediction or via temporal prediction withreference to one or two previously coded reference pictures.

The video encoder (603) may perform coding operations according to apredetermined video coding technology or standard, such as ITU-T Rec.H.265. In its operation, the video encoder (603) may perform variouscompression operations, including predictive coding operations thatexploit temporal and spatial redundancies in the input video sequence.The coded video data, therefore, may conform to a syntax specified bythe video coding technology or standard being used.

In an embodiment, the transmitter (640) may transmit additional datawith the encoded video. The source coder (630) may include such data aspart of the coded video sequence. Additional data may comprisetemporal/spatial/SNR enhancement layers, other forms of redundant datasuch as redundant pictures and slices, SEI messages, VUI parameter setfragments, and so on.

A video may be captured as a plurality of source pictures (videopictures) in a temporal sequence. Intra-picture prediction (oftenabbreviated to intra prediction) makes use of spatial correlation in agiven picture, and inter-picture prediction makes uses of the (temporalor other) correlation between the pictures. In an example, a specificpicture under encoding/decoding, which is referred to as a currentpicture, is partitioned into blocks. When a block in the current pictureis similar to a reference block in a previously coded and still bufferedreference picture in the video, the block in the current picture can becoded by a vector that is referred to as a motion vector. The motionvector points to the reference block in the reference picture, and canhave a third dimension identifying the reference picture, in casemultiple reference pictures are in use.

In some embodiments, a bi-prediction technique can be used in theinter-picture prediction. According to the bi-prediction technique, tworeference pictures, such as a first reference picture and a secondreference picture that are both prior in decoding order to the currentpicture in the video (but may be in the past and future, respectively,in display order) are used. A block in the current picture can be codedby a first motion vector that points to a first reference block in thefirst reference picture, and a second motion vector that points to asecond reference block in the second reference picture. The block can bepredicted by a combination of the first reference block and the secondreference block.

Further, a merge mode technique can be used in the inter-pictureprediction to improve coding efficiency.

According to some embodiments of the disclosure, predictions, such asinter-picture predictions and intra-picture predictions are performed inthe unit of blocks. For example, according to the HEVC standard, apicture in a sequence of video pictures is partitioned into coding treeunits (CTU) for compression, the CTUs in a picture have the same size,such as 64×64 pixels, 32×32 pixels, or 16×16 pixels. In general, a CTUincludes three coding tree blocks (CTBs), which are one luma CTB and twochroma CTBs, Each CTU can be recursively quadtree split into one ormultiple coding units (CUs). For example, a CTU of 64×64 pixels can besplit into one CU of 64×64 pixels, or 4 CUs of 32×32 pixels, or 16 CUsof 16×16 pixels. In an example, each CU is analyzed to determine aprediction type for the CU, such as an inter prediction type or an intraprediction type. The CU is split into one or more prediction units (PUs)depending on the temporal and/or spatial predictability. Generally, eachPU includes a luma prediction block (PB), and two chroma PBs. In anembodiment, a prediction operation in coding (encoding/decoding) isperformed in the unit of a prediction block. Using a luma predictionblock as an example of a prediction block, the prediction block includesa matrix of values (e.g., luma values) for pixels, such as 8×8 pixels,16×16 pixels, 8×16 pixels. 16×8 pixels, and the like.

FIG. 7 shows a diagram of a video encoder (703) according to anotherembodiment of the disclosure. The video encoder (703) is configured toreceive a processing block (e.g., a prediction block) of sample valueswithin a current video picture in a sequence of video pictures, andencode the processing block into a coded picture that is part of a codedvideo sequence. In an example, the video encoder (703) is used in theplace of the video encoder (403) in the FIG. 4 example.

In an HEW example, the video encoder (703) receives a matrix of samplevalues for a processing block, such as a prediction block of 8×8samples, and the like. The video encoder (703) determines whether theprocessing block is best coded using intra mode, inter mode, orbi-prediction mode using, for example, rate-distortion optimization.When the processing block is to be coded in intra mode, the videoencoder (703) may use an intra prediction technique to encode theprocessing block into the coded picture; and when the processing blockis to be coded in inter mode or bi-prediction mode, the video encoder(703) may use an inter prediction or bi-prediction technique,respectively, to encode the processing block into the coded picture. Incertain video coding technologies, merge mode can be an inter pictureprediction submode where the motion vector is derived from one or moremotion vector predictors without the benefit of a coded motion vectorcomponent outside the predictors. In certain other video codingtechnologies, a motion vector component applicable to the subject blockmay be present. In an example, the video encoder (703) includes othercomponents, such as a mode decision module (not shown) to determine themode of the processing blocks.

In the FIG. 7 example, the video encoder (703) includes the interencoder (730), an intra encoder (722), a residue calculator (723), aswitch (726), a residue encoder (724), a general controller (721), andan entropy encoder (725) coupled together as shown in FIG. 7.

The inter encoder (730) is configured to receive the samples of thecurrent block (e.g., a processing block), compare the block to one ormore reference blocks in reference pictures (e.g., blocks in previouspictures and later pictures), generate inter prediction information(e.g., description of redundant information according to inter encodingtechnique, motion vectors, merge mode information), and calculate interprediction results (e.g., predicted block) based on the inter predictioninformation using any suitable technique. In some examples, thereference pictures are decoded reference pictures that are decoded basedon the encoded video information.

The intra encoder (722) is configured to receive the samples of thecurrent block (e.g., a processing block), in some cases compare theblock to blocks already coded in the same picture, generate quantizedcoefficients after transform, and in some cases also intra predictioninformation (e.g., an intra prediction direction information accordingto one or more intra encoding techniques). In an example, the intraencoder (722) also calculates intra prediction results (e.g., predictedblock) based on the intra prediction information and reference blocks inthe same picture.

The general controller (721) is configured to determine general controldata and control other components of the video encoder (703) based onthe general control data. In an example, the general controller (721)determines the mode of the block, and provides a control signal to theswitch (726) based on the mode. For example, when the mode is the intramode, the general controller (721) controls the switch (726) to selectthe intra mode result for use by the residue calculator (723), andcontrols the entropy encoder (725) to select the intra predictioninformation and include the intra prediction information in thebitstream; and when the mode is the inter mode, the general controller(721) controls the switch (726) to select the inter prediction resultfor use by the residue calculator (723), and controls the entropyencoder (725) to select the inter prediction information and include theinter prediction information in the bitstream.

The residue calculator (723) is configured to calculate a difference(residue data) between the received block and prediction resultsselected from the intra encoder (722) or the inter encoder (730). Theresidue encoder (724) is configured to operate based on the residue datato encode the residue data to generate the transform coefficients. In anexample, the residue encoder (724) is configured to convert the residuedata from a spatial domain to a frequency domain, and generate thetransform coefficients. The transform coefficients are then subject toquantization processing to obtain quantized transform coefficients. Invarious embodiments, the video encoder (703) also includes a residuedecoder (728). The residue decoder (728) is configured to performinverse-transform, and generate the decoded residue data. The decodedresidue data can be suitably used by the intra encoder (722) and theinter encoder (730). For example, the inter encoder (730) can generatedecoded blocks based on the decoded residue data and inter predictioninformation, and the intra encoder (722) can generate decoded blocksbased on the decoded residue data and the intra prediction information.The decoded blocks are suitably processed to generate decoded picturesand the decoded pictures can be buffered in a memory circuit (not shown)and used as reference pictures in some examples.

The entropy encoder (725) is configured to format the bitstream toinclude the encoded block. The entropy encoder (725) is configured toinclude various information according to a suitable standard, such asthe HEVC standard. In an example, the entropy encoder (725) isconfigured to include the general control data, the selected predictioninformation (e.g., intra prediction information or inter predictioninformation), the residue information, and other suitable information inthe bitstream. Note that, according to the disclosed subject matter,when coding a block in the merge submode of either inter mode orbi-prediction mode, there is no residue information.

FIG. 8 shows a diagram of a video decoder (810) according to anotherembodiment of the disclosure. The video decoder (810) is configured toreceive coded pictures that are part of a coded video sequence, anddecode the coded pictures to generate reconstructed pictures. In anexample, the video decoder (810) is used in the place of the videodecoder (410) in the FIG. 4 example.

In the FIG. 8 example, the video decoder (810) includes an entropydecoder (871), an inter decoder (880), a residue decoder (873), areconstruction module (874), and an intra decoder (872) coupled togetheras shown in FIG. 8.

The entropy decoder (871) can be configured to reconstruct, from thecoded picture, certain symbols that represent the syntax elements ofwhich the coded picture is made up. Such symbols can include, forexample, the mode in which a block is coded (such as, for example, intramode, inter mode, bi-predicted mode, the latter two in merge submode oranother submode), prediction information (such as, for example, intraprediction information or inter prediction information) that canidentify certain sample or metadata that is used for prediction by theintra decoder (872) or the inter decoder (880), respectively, residualinformation in the form of, for example, quantized transformcoefficients, and the like. In an example, when the prediction mode isinter or bi-predicted mode, the inter prediction information is providedto the inter decoder (880); and when the prediction type is the intraprediction type, the intra prediction information is provided to theintra decoder (872). The residual information can be subject to inversequantization and is provided to the residue decoder (873).

The inter decoder (880) is configured to receive the inter predictioninformation, and generate inter prediction results based on the interprediction information.

The intra decoder (872) is configured to receive the intra predictioninformation, and generate prediction results based on the intraprediction information.

The residue decoder (873) is configured to perform inverse quantizationto extract de-quantized transform coefficients, and process thede-quantized transform coefficients to convert the residual from thefrequency domain to the spatial domain. The residue decoder (873) mayalso require certain control information (to include the QuantizerParameter (QP)), and that information may be provided by the entropydecoder (871) (data path not depicted as this may be low volume controlinformation only).

The reconstruction module (874) is configured to combine, in the spatialdomain, the residual as output by the residue decoder (873) and theprediction results (as output by the inter or intra prediction modulesas the case may be) to form a reconstructed block, that may be part ofthe reconstructed picture, which in turn may be part of thereconstructed video. It is noted that other suitable operations, such asa deblocking operation and the like, can be performed to improve thevisual quality.

It is noted that the video encoders (403), (603), and (703), and thevideo decoders (410), (510), and (810) can be implemented using anysuitable technique. In an embodiment, the video encoders (403), (603),and (703), and the video decoders (410), (510), and (810) can beimplemented using one or more integrated circuits. In anotherembodiment, the video encoders (403), (603), and (603), and the videodecoders (410), (510), and (810) can be implemented using one or moreprocessors that execute software instructions.

Aspects of the disclosure provide techniques for search range adjustmentfor intra picture block compensation.

Block based compensation can be used for inter prediction and intraprediction. For the inter prediction, block based compensation from adifferent picture is known as motion compensation. Block basedcompensation can also be done from a previously reconstructed areawithin the same picture, such as in intra prediction. The block basedcompensation from reconstructed area within the same picture is referredto as intra picture block compensation, current picture referencing(CPR), or intra block copy (IBC). A displacement vector that indicatesan offset between a current block and a reference block (also referredto as a prediction block) in the same picture is referred to as a blockvector (By) where the current block can be encoded/decoded based on thereference block. Different from a motion vector in motion compensation,which can be at any value (positive or negative, at either x or ydirection), a BV has a few constraints to ensure that the referenceblock is available and already reconstructed. Also, in some examples,for parallel processing consideration, some reference area that is tileboundary, slice boundary, or wavefront ladder shape boundary isexcluded.

The coding of a block vector could be either explicit or implicit. Inthe explicit mode, a BV difference between a block vector and itspredictor is signaled. In the implicit mode, the block vector isrecovered from a predictor (referred to as block vector predictor)without using the BV difference, in a similar way as a motion vector inmerge mode. The resolution of a block vector, in some implementations,is restricted to integer positions. In other systems, the block vectoris allowed to point to fractional positions.

In some examples, the use of intra block copy at a block level can besignaled using a block level flag, such as an IBC flag. In anembodiment, the block level flag is signaled when the current block iscoded explicitly. In some examples, the use of intra block copy at ablock level can be signaled using a reference index approach. Thecurrent picture under decoding is then treated as a reference picture ora special reference picture. In an example, such a reference picture isput in the last position of a list of reference pictures. The specialreference picture is also managed together with other temporal referencepictures in a buffer, such as a decoded picture buffer (DPB).

There are also some variations for intra block copy, such as flippedintra block copy (the reference block is flipped horizontally orvertically before used to predict a current block), or line based intrablock copy (each compensation unit inside an M×N coding block is an M×1or 1×N line).

As described above, a BV of a current block under reconstruction in apicture can have certain constraints, and thus, a reference block forthe current block is within a search range. The search range refers to apart of the picture from which the reference block can be selected. Forexample, the search range may be within certain portions of areconstructed area in the picture. A size, a position, a shape, and/orthe like of the search range can be constrained. Alternatively, the BVcan be constrained. In an example, the BV is a two-dimensional vectorincluding an x and a y component, and at least one of the x and ycomponents can be constrained. Constraints can be specified with respectto the BV, the search range, or a combination of the BV and the searchrange. In various examples, when certain constraints are specified withrespect to the BV, the search range is constrained accordingly.Similarly, when certain constraints are specified with respect to thesearch range, the BV is constrained accordingly.

FIG. 9 shows an example of intra block copy according to an embodimentof the disclosure. A current picture (900) is to be reconstructed underdecoding. The current picture (900) includes a reconstructed area (910)(grey area) and a to-be-decoded area (920) (white area). A current block(930) is under reconstruction by a decoder. The current block (930) canbe reconstructed from a reference block (940) that is in thereconstructed area (910). A position offset between the reference block(940) and the current block (930) is referred to as a block vector (950)(or BV (950)). In the FIG. 9 example, a search range (960) is within thereconstructed area (910), the reference block (940) is within the searchrange (960), and the block vector (950) is constrained to point to thereference block (940) within the search range (960).

Various constraints can be applied to a BV and/or a search range. In anembodiment, a search range for a current block under reconstruction in acurrent CTB is constrained to be within the current CTB.

In an embodiment, an effective memory requirement to store referencesamples to be used in intra block copy is one CTB size. In an example,the CTB size is 128×128 samples. A current CTB includes a current regionunder reconstruction. The current region has a size of 64×64 samples.Since a reference memory can also store reconstructed samples in thecurrent region, the reference memory can store 3 more regions of 64×64samples when a reference memory size is equal to the CTB size of 128×128samples. Accordingly, a search range can include certain parts of apreviously reconstructed CTB while a total memory requirement forstoring reference samples is unchanged (such as 1 CTB size of 128×128samples or 4 64×64 reference samples in total). In an example, thepreviously reconstructed CTB is a left neighbor of the current CTB, suchas shown in FIG. 10.

FIG. 10 shows an example of intra block copy according to an embodimentof the disclosure. A current picture (1001) includes a current CTB(1015) under reconstruction and a previously reconstructed CTB (1010)that is a left neighbor of the current CTB (1015). CTBs in the currentpicture (1001) have a CTB size, such as 128×128 samples, and a CTBwidth, such as 128 samples. The current cm (1015) includes 4 regions(1016)-(1019), where the current region (1016) is under reconstruction.The current region (1016) includes a plurality of coding blocks(1021)-(1029). Similarly, the previously reconstructed CTB (1010)includes 4 regions (1011)-(1014). The coding blocks (1021)-(1025) arereconstructed, the current block (1026) is under reconstruction, and thecoding blocks (1026)-(1027) and the regions (1017)-(1019) are to bereconstructed.

The current region (1016) has a collocated region (i.e., the region(1011), in the previously reconstructed CTB (1010)). A relative positionof the collocated region (1011) with respect to the previouslyreconstructed CTB (1010) can be identical to a relative position of thecurrent region (1016) with respect to the current CTB (1015). In theexample illustrated in FIG. 10, the current region (1016) is a top leftregion in the current CTB (1015), and thus, the collocated region (1011)is also a top left region in the previously reconstructed CTB (1010).Since a position of the previously reconstructed CTB (1010) is offsetfrom a position of the current CTB (1015) by the CTB width, a positionof the collocated region (1011) is offset from a position of the currentregion (1016) by the CTB width.

In an embodiment, a collocated region of the current region (1016) is ina previously reconstructed CTB where a position of the previouslyreconstructed CTB is offset by one or multiples of the CTB width fromthe positon of the current CTB (1015), and thus, a position of thecollocated region is also offset by a corresponding one or multiples ofthe CTB width from the position of the current region (1016). Theposition of the collocated region can be left shifted, up shifted, orthe like from the current region (1016).

As described above, a size of a search range for the current block(1026) is constrained by the CTB size. In the FIG. 10 example, thesearch range can include the regions (1012)-(1014) in the previouslyreconstructed CTB (1010) and a portion of the current region (1016) thatis already reconstructed, such as the coding blocks (1021)-(1025). Thesearch range further excludes the collocated region (1011) so that thesize of the search range is within the CTB size. Referring to FIG. 10, areference block (1091) is located in the region (1014) of the previouslyreconstructed CTB (1010). A block vector (1020) indicates an offsetbetween the current block (1026) and the respective reference block(1091). The reference block (1091) is in the search range.

The example illustrated in FIG. 10 can be suitably adapted to otherscenarios where a current region is located at another location in thecurrent CTB (1015). In an example, when a current block is in the region(1017), a collocated region for the current block is the region (1012).Therefore, a search range can include the regions (1013)-(1014), theregion (1016), and a portion of the region (1017) that is alreadyreconstructed. The search range further excludes the region (1011) andthe collocated region (1012) so that the size of the search range iswithin the CTB size. In an example, when a current block is in theregion (1018), a collocated region for the current block is the region(1013). Therefore, a search range can include the region (1014), theregions (1016)-(1017), and a portion of the region (1018) that isalready reconstructed. The search range further excludes the regions(1011)-(1012) and the collocated region (1013) so that the size of thesearch range is within the CTB size. In an example, when a current blockis in the region (1019), a collocated region for the current block isthe region (1014). Therefore, a search range can include the regions(1016)-(1018), and a portion of the region (1019) that is alreadyreconstructed. The search range further excludes the previouslyreconstructed CTB (1010) so that the size of the search range is withinthe CTB size.

In the above description, a reference block can be in the previouslyreconstructed CTB (1010) or the current CTB (1015).

In an embodiment, a search range can be specified as below. In anexample, a current picture is a luma picture and a current CTB is a lumaCTB including a plurality of luma samples and a block vector mvLsatisfies the following constraints for bitstream conformance.

The constraints include first conditions that a reference block for thecurrent block is already reconstructed. When the reference block has arectangular shape, a reference block availability checking process canbe implemented to check whether a top left sample and a bottom rightsample of the reference block are reconstructed. When both the top leftsample and the bottom right sample of the reference block arereconstructed, the reference block is determined to be reconstructed.

For example, when a derivation process for reference block availabilityis invoked with a position (xCurr, yCurr) of a top left sample of thecurrent block set to be (xCb, yCb) and a position (xCb+(mvL[0]>>4),yCb+(mvL[1]>>4)) of the top left sample of the reference block asinputs, an output is equal to TRUE when the top left sample of thereference block is reconstructed where the block vector mvL is atwo-dimensional vector having a x component mvL[0] and a y componentmv[4].

Similarly, when a derivation process for block availability is invokedwith the position (xCurr, yCurr) of the top left sample of the currentblock set to be (xCb, yCb) and a position (xCb+(mvL[0]>>4)+cbWidth−1,yCb+(mvL[1]>>4)+cbHeight−1) of the bottom right sample of the referenceblock as inputs, an output is equal to TRUE when the bottom right sampleof the reference block is reconstructed. The parameters cbWidth andcbHeight represent a width and a height of the reference block.

The constraints can also include at least one of the following secondconditions: 1) a value of (mvL[0]>>4)+cbWidth is less than or equal to0, which indicates that the reference block is to the left of thecurrent block and does not overlap with the current block; 2) a value of(mvL[1]>>4)+cbHeight is less than or equal to 0, which indicates thatthe reference block is above the current block and does not overlap withthe current block.

The constraints can also include that the following third conditions aresatisfied by the block vector mvL:(yCb+(mvL[1]>>4))>>Ctb Log 2SizeY=yCb>>Ctb Log 2SizeY  (1)(yCb+(mvL[1]>>4+cbHeight−1)>>Ctb Log 2SizeY=yCb>>Ctb Log 2Size  (2)(xCb+(mvL[0]>>4))>>Ctb Log 2SizeY>>(xCb>>Ctb Log 2SizeY)−1  (3)(xCb+(mvL[0]:>>4)+cbWidth−1)=>>Ctb Log 2SizeY<=(xCb>>Ctb Log2SizeY)  (4)where the parameters Ctb Log 2SizeY represents the CTB width in log 2form. For example, when the CTB width is 128 samples, Ctb Log 2SizeY is7. Eqs. (1)-(2) specify that a CTB including the reference block is in asame CTB row as the current CTB (i.e., the previously reconstructed CTB(1010) is in a same row as the current CTB (1015) when the referenceblock is in the previously reconstructed CTB (1010)). Eqs. (3)-(4)specify that the CTB including the reference block is either in a leftCTB column of the current CTB or a same CTB column as the current CTB.The third conditions as described by Eqs. (1)-(4) specify that the CTBincluding the reference block is either the current CTB, such as thecurrent CTB (1015), or a left neighbor, such as the previouslyreconstructed CTB (1010), of the current CTB, similarly to thedescription with reference to FIG. 10.

The constraints can further include fourth conditions: when thereference block is in the left neighbor of the current CTB, a collocatedregion for the reference block is not reconstructed (i.e., no samples inthe collocated region have been reconstructed). Further, the collocatedregion for the reference block is in the current CTB. In the FIG. 10example, a collocated region for the reference block (1091) is theregion (1019) that is offset by the CTB width from the region (1014)where the reference block (1091) is located and the region (1019) hasnot been reconstructed. Therefore, the block vector (1020) and thereference block (1091) satisfy the fourth conditions described above.

In an example, the fourth conditions can be specified as below: when(xCb+(mvL[0]>>4))>>Ctb Log 2SizeY is equal to (xCb>>Ctb Log 2SizeY)−1,the derivation process for reference block availability is invoked withthe position of the current block (xCurr, yCurr) set to be (xCb, yCb)and a position (((xCb+(mvL[0]>>4)+CtbSizeY)>>(Ctb Log 2SizeY−1))<<(CtbLog 2SizeY 1), ((yCb+(mvL[1]>>4))>>(Ctb Log 2SizeY−1))<<(Ctb Log2SizeY−1)) as inputs, an output is equal to FALSE indicating that thecollocated region is not reconstructed, such as shown in FIG. 10.

The constraints for the search range and/or the block vector can includea suitable combination of the first, second, third, and fourthconditions described above. In an example, the constraints include thefirst, second, third, and fourth conditions, such as shown in FIG. 10.In an example, the first, second, third, and/or fourth conditions can bemodified and the constraints include the modified first, second, third,and/or fourth conditions.

According to the fourth conditions, when one of the coding blocks(1022)-(1029) is a current block, a reference block cannot be in theregion (1011), and thus, a search range for the one of the coding blocks(1022)-(1029) excludes the region (1011). The reasons why the region(1011) is excluded are specified below: if the reference block is in theregion (1011), then a collocated region for the reference block is theregion (1016), however, at least samples in the coding block (1021) havebeen reconstructed, and thus, the fourth conditions are violated. On theother hand, for a coding block to be reconstructed first in a currentregion, such as a coding block (1121) in a region (1116) in FIG. 11, thefourth conditions does not prevent a reference block to be in the region(1111) because a collocated region (1116) for the reference block hasnot been reconstructed yet.

FIG. 11 shows an example of intra block copy according to an embodimentof the disclosure. A current picture (1101) includes a current CTB(1115) under reconstruction and a previously reconstructed CTB (1110)that is a left neighbor of the current CTB (1115). CTBs in the currentpicture (1101) have a CTB size and a CTB width. The current CTB (1115)includes 4 regions (1116)-(1119) where the current region (1116) isunder reconstruction. The current region (1116) includes a plurality ofcoding blocks (1121)-(1129). Similarly, the previously reconstructed CTB(1110) includes 4 regions (1111)-(1114). The current block (1121) underreconstruction is to be reconstructed first in the current region (1116)and the coding blocks (1122)-(1129) are to be reconstructed. In anexample, the CTB size is 128×128 samples, each of the regions(1111)-(1114) and (1116)-(1119) is 64×64 samples. A reference memorysize is equal to the CTB size and is 128×128 samples, and thus, thesearch range, when bounded by the reference memory size, includes 3regions and a portion of an additional region.

Similarly as described with reference to FIG. 10, the current region(1116) has a collocated region (i.e., the region (1111) in thepreviously reconstructed CTB (1110)). According to the fourth conditionsdescribed above, a reference block for the current block (1121) can bein the region (1111), and thus, a search range can include the regions(1111)-(1114). For example, when the reference block is in the region(1111), a collocated region of the reference block is the region (1116),where no samples in the region (1116) have been reconstructed prior tothe reconstruction of the current block (1121). However, as describedwith reference to FIG. 10 and the fourth conditions, for example, afterthe reconstruction of the coding block (1121), the region (1111) is nolonger available to be included in a search range for reconstructing thecoding block (1122). Therefore, a tight synchronization and timingcontrol of the reference memory buffer is to be used and can bechallenging.

According to some embodiments, when a current block is to bereconstructed first in a current region of a current CTB, a search rangecan exclude a collocated region of the current region that is in apreviously reconstructed CTB where the current CTB and the previouslyreconstructed CTB are in a same current picture. A block vector can bedetermined such that a reference block is in the search range thatexcludes the collocated region in the previously reconstructed CTB. Inan embodiment, the search range includes coding blocks that arereconstructed after the collocated region and before the current blockin a decoding order.

In the descriptions below, a CTB size can vary and a maximum CTB size isset to be identical to a reference memory size. In an example, thereference memory size or the maximum CTB size is 128×128 samples. Thedescriptions can be suitably adapted to other reference memory sizes ormaximum CTB sizes.

In an embodiment, the CTB size is equal to the reference memory size.The previously reconstructed CTB is a left neighbor of the current CTB,a position of the collocated region is offset by a CTB width from aposition of the current region, and the coding blocks in the searchrange are in at least one of: the current CTB and the previouslyreconstructed CTB.

FIGS. 12A-12D show examples of intra block copy according to anembodiment of the disclosure. Referring to FIGS. 12A-D, a currentpicture (1201) includes a current CTB (1215) under reconstruction and apreviously reconstructed CTB (1210) that is a left neighbor of thecurrent CTB (1215). CTBs in the current picture (1201) have a CTB sizeand a CTB width. The current CTB (1215) includes 4 regions(1216)-(1219). Similarly, the previously reconstructed CTB (1210)includes 4 regions (1211)-(1214). In an embodiment, the CTB size is amaximum CTB size and is equal to a reference memory size. In an example,the CTB size and the reference memory size are 128 by 128 samples, andthus, each of the regions (1211)-(1214) and (1216)-(1219) has a size of64 by 64 samples.

In the examples illustrated in FIGS. 12A-D, the current CTB (1215)includes a top left region, a top right region, a bottom left region,and a bottom right region that correspond to the regions (1216)-(1219),respectively. The previously reconstructed CTB (1210) includes a topleft region, a top right region, a bottom left region, and a bottomright region that correspond to the regions (1211)-(1214), respectively.

Referring to FIG. 12A, the current region (1216) is underreconstruction. The current region (1216) includes a plurality of codingblocks (1221)-(1229). The current block (1221) is to be reconstructedfirst in the current region (1216). The current region (1216) has acollocated region, i.e., the region (1211), in the previouslyreconstructed CTB (1210). According to some embodiments, a search rangefor the current block (1221) excludes the collocated region (1211) wherethe current block (1221) is to be reconstructed first in the currentregion (1216). Therefore, a tight synchronization and timing control ofa reference memory buffer is not necessary. Otherwise, when the currentblock (1221) is to be reconstructed first in the current region (1216)and the search range for the current block (1221) includes thecollocated region (1211) and the regions (1212)-(1214), samples of thecollocated region (1211) can be used to predict the current block(1221). For example, the samples can be from a collocated block of thecurrent block (1221) in the previously reconstructed CTB (1210), then aprocessing order in the reference memory buffer can include: reading (orobtaining) a sample from a position x in the reference memory buffer,performing prediction for a sample in the current block (1221) by usingthe sample from the position x, adding a residue to the prediction, andthen writing back the reconstructed sample to the position x in thereference memory buffer. The writing and reading processes to the samereference memory location x can require a tight synchronization, whichmay not be preferred in some examples. The search range includes theregions (1212)-(1214) of the previously reconstructed CTB (1210) thatare reconstructed after the collocated region (1211) and before thecurrent block (1221) in a decoding order.

Referring to FIG. 12A, a position of the collocated region (1211) isoffset by the CTB width, such as 128 samples, from a position of thecurrent region (1216). For example, the position of the collocatedregion (1211) is left shifted by 128 samples from the position of thecurrent region (1216).

Referring again to FIG. 12A, when the current region (1216) is the topleft region of the current CTB (1215), the collocated region (1211) isthe top left region of the previously reconstructed CTB (1210), and thesearch region excludes the top left region of the previouslyreconstructed CTB.

Referring to FIG. 12B, the current region (1217) is underreconstruction. The current region (1217) includes a plurality of codingblocks (1241)-(1249). The current block (1241) is to be reconstructedfirst in the current region (1217). The current region (1217) has acollocated region (i.e., the region (1212), in the previouslyreconstructed CTB (1210)). According to aspects of the disclosure, asearch range for the current block (1241) excludes the collocated region(1212) where the current block (1241) is to be reconstructed first inthe current region (1217). Therefore, a tight synchronization and timingcontrol of a reference memory buffer is not necessary. The search rangeincludes the regions (1213)-(1214) of the previously reconstructed CTB(1210) and the region (1216) in the current CTB (1215) that arereconstructed after the collocated region (1212) and before the currentblock (1241). The search range further excludes the region (1211) due toconstraint of the reference memory size (i.e., one CTB size). Similarly,a position of the collocated region (1212) is offset by the CTB width,such as 128 samples, from a position of the current region (1217).

In the FIG. 12B example, the current region (1217) is the top rightregion of the current CTB (1215), the collocated region (1212) is alsothe top right region of the previously reconstructed CTB (1210), and thesearch region excludes the top right region of the previouslyreconstructed CTB (1210).

Referring to FIG. 12C, the current region (1218) is underreconstruction. The current region (1218) includes a plurality of codingblocks (1261)-(1269). The current block (1261) is to be reconstructedfirst in the current region (1218). The current region (1218) has acollocated region (i.e., the region (1213)), in the previouslyreconstructed CTB (1210). According to aspects of the disclosure, asearch range for the current block (1261) excludes the collocated region(1213) where the current block (1261) is to be reconstructed first inthe current region (1218). Therefore, a tight synchronization and timingcontrol of a reference memory buffer is not necessary. The search rangeincludes the region (1214) of the previously reconstructed CTB (1210)and the regions (1216)-(1217) in the current CTB (1215) that arereconstructed after the collocated region (1213) and before the currentblock (1261). Similarly, the search range further excludes the regions(1211)-(1212) due to constraint of the reference memory size. A positionof the collocated region (1213) is offset by the CTB width, such as 128samples, from a position of the current region (1218). In the FIG. 12Cexample, when the current region (1218) is the bottom left region of thecurrent CTB (1215), the collocated region (1213) is also the bottom leftregion of the previously reconstructed CTB (1210), and the search regionexcludes the bottom left region of the previously reconstructed CTB(1210).

Referring to FIG. 12D, the current region (1219) is underreconstruction. The current region (1219) includes a plurality of codingblocks (1281)-(1289). The current block (1281) is to be reconstructedfirst in the current region (1219). The current region (1219) has acollocated region (i.e., the region (1214)), in the previouslyreconstructed CTB (1210). According to aspects of the disclosure, asearch range for the current block (1281) excludes the collocated region(1214) where the current block (1281) is to be reconstructed first inthe current region (1219). Therefore, a tight synchronization and timingcontrol of a reference memory buffer is not necessary. The search rangeincludes the regions (1216)-(1218) in the current CTB (1215) that arereconstructed after the collocated region (1214) and before the currentblock (1281) in a decoding order. The search range excludes the regions(1211)-(1213) due to constraint of the reference memory size, and thus,the search range excludes the previously reconstructed CTB (1210).Similarly, a position of the collocated region (1214) is offset by theCTB width, such as 128 samples, from a position of the current region(1219). In the FIG. 12D example, when the current region (1219) is thebottom right region of the current CTB (1215), the collocated region(1214) is also the bottom right region of the previously reconstructedCTB (1210) and the search region excludes the bottom right region of thepreviously reconstructed CTB (1210).

As described above with reference to FIGS. 12A-12D, a search range and ablock vector mvL of a current block satisfy the modified fourthconditions where the current block is to be reconstructed first in acurrent region of a current CTB. In some embodiments, the modifiedfourth conditions are specified as: when (xCb+(mvL[0]>>4))>>Ctb Log2SizeY is equal to (xCb>>Ctb Log 2SizeY)−1, the derivation process forreference block availability is invoked with the position of the currentblock (xCurr, yCurr) set to be (xCb, yCb) and a position(((xCb+mvL[0]>>4)+CtbSizeY)>>(Ctb Log 2SizeY−1))<<(Ctb Log 2SizeY−1),((yCb+(mvL[1]>>4))>>(Ctb Log 2SizeY−1))<<(Ctb Log 2SizeY−1)) as inputs,an output is equal to FALSE indicating that the collocated region is notreconstructed.

Further, the modified fourth conditions include an additional condition:a position (((xCb+(mvL[0]=4)+CtbSizeY)>>(Ctb Log 2SizeY−1))<<(Ctb Log2SizeY−1), ((yCb+(mvL[1]>>4))>>(Ctb Log 2SizeY−1))<<(Ctb Log 2SizeY−1))is not equal to (xCb, yCb). As described with reference to FIG. 10, aposition of a top left sample of the current block is represented by(xCb, yCb), a position of a top left sample of a reference block isrepresented by (xCb+(mvL[0]>>4), yCb (mvL[1]>>4)), and thus, a positionof a top left sample of a collocated region of the reference block isrepresented by the position (((xCb+(mvL[0]>>4)+CtbSizeY)>>(Ctb Log2SizeY−1))<<(Ctb Log 2SizeY−1), ((yCb+(mvL[1]>>4))>>(Ctb Log2SizeY−1))<<(Ctb Log 2SizeY−1)), where the collocated region of thereference block is in the current CTB. The additional condition ensuresthat the position of the top left sample of the collocated region of thereference block is not equal to the position of the top left sample ofthe current block. In this regard, when the current block is to bereconstructed first in the current region, the reference block cannot bein a collocated region of the current region. Otherwise, the additionalcondition is not satisfied. Accordingly, the search range excludes thecollocated region of the current region.

In the examples illustrated in FIGS. 12A-12D, the search range and theblock vector also satisfy the first, the second, and the thirdconditions described with reference to FIG. 10.

As described above, when a current block is to be reconstructed first ina current region of a current CTB, a search range can exclude acollocated region of the current region that is in a previouslyreconstructed CTB where the current CTB and the previously reconstructedCTB are in a same current picture. According to aspects of thedisclosure, when a CTB size is less than a reference memory size, aposition of the collocated region can be offset by multiples of the CTBwidth from a position of the current region, and coding blocks in thesearch range are in at least one of: the current CTB, the previouslyreconstructed CTB, and one or more reconstructed CTBs between thecurrent CTB and the previously reconstructed CTB. The descriptions withreference to FIGS. 12A-12D can be suitably adapted when the CTB size isless than the reference memory size such as shown in FIG. 13.

FIG. 13 shows an example of intra block copy having a search range thatis larger than a CTB size according to an embodiment of the disclosure.A current picture (1301) includes a current CTB (1315) underreconstruction and multiple previously reconstructed CTBs (1310) and(1321)-(1323). CTBs in the current picture (1301) have a CTB size and aCTB width. The current CTB (1315) includes 4 regions (1316)-(1319).Similarly, the previously reconstructed CTB (1310) includes 4 regions(1311)-(1314). In an example, a reference memory size is 128×128 samplesand can be equal to a maximum CTB size, the CTB size being smaller thanthe reference memory size or the maximum CTB size is 64 by 64 samples,and each of the regions (1311)-(1314) and (1316)-(1319) has a size of 32by 32 samples. A ratio N is a ratio of the reference memory size overthe CTB size.

The current CTB (1315) includes a top left region, a top right region, abottom left region, and a bottom right region that correspond to theregions (1316)-(1319), respectively. The previously reconstructed CTB(1310) includes a top left region, a top right region, a bottom leftregion, and a bottom right region that correspond to the regions(1311)-(1314), respectively.

The current region (1317) is under reconstruction. The current region(1317) includes a plurality of coding blocks A-I. The current block A isto be reconstructed first in the current region (1317). The currentregion (1317) has a collocated region (1312) in the previouslyreconstructed CTB (1310). According to aspects of the disclosure, asearch range for the current block A excludes the collocated region(1312). The search range includes the regions (1313)-(1314) of thepreviously reconstructed CTB (1310), the CTBs (1321)-(1323), and theregion (1316) that are reconstructed after the collocated region (1312)and before the current block A. Therefore, a left most CTB that thesearch range can include is offset by N of the CTB width from thecurrent CTB (1315). A position of the collocated region (1312) is alsooffset by N of the CTB width from a position of the current region(1317). In the FIG. 13 example, the ratio N is 4, the left most CTB isthe previously reconstructed CTB (1310) that is offset by 4 of the CTBwidth from the current CTB (1315). The position of the collocated region(1312) is left shifted by 256 samples, i.e., 4 of the CTB width (64samples), from the position of the current region (1317).

As shown in the FIG. 13 example, when the current region (1317) is thetop right region of the current CTB (1315), the collocated region (1312)is also the top right region of the previously reconstructed CTB (1310)and the search region excludes the top right region of the previouslyreconstructed CTB (1310).

The descriptions with reference to FIG. 13 can be suitably adapted whena current block is to be reconstructed first in another region, such asthe region (1316), the region (1318), or the region (1319). For purposesof brevity, the detailed description is omitted.

When a CTB size is smaller than a reference memory size, for example,the CTB size is 64×64 samples and the reference memory size is 128×128samples, different embodiments other than the FIG. 13 example can beimplemented as below. In the embodiments below, a current block to bereconstructed using the IBC mode is in a current region of a current CTBunder reconstruction. A reference block of the current block is in asearch range. The ratio N of the reference memory size over the CTB sizeis larger than 1. N previously reconstructed CTBs are left shifted by N,(N−1), . . . , and 1 of the CTB width from the current CTB,respectively. The search range can include at least one of: the currentCTB, a left most CTB (i.e., the CTB left shifted by N of the CTB width),and (N−1) previously reconstructed CTBs (also referred to as the (N−1)CTBs) between the left most CTB and the current CTB.

In a first embodiment, the search range for the current block is withinthe current CTB and a previously reconstructed CTB that is a leftneighbor of the current CTB. Because the reference memory size is atleast 2 of the CTB size, each coding block of the left neighbor can beavailable as the reference block, and thus, no additional check forreference block availability is necessary. In an example, the currentblock is reconstructed first in the current region. In an example, thecurrent block is reconstructed after a previously reconstruced codingblock in the current region.

In a second embodiment, the search range is extended to include the(N−1) CTBs between the left most CTB and the current CTB. Accordingly,the search range includes the (N−1) CTBs and excludes the left most CTB.The search range can further include reconstructed portion in thecurrent CTB. Because the reference memory size is N of the CTB size, asize of the search range is within the reference memory size, and thus,no additional check for reference block availability is necessary whenthe (N−1) CTBs and the current CTB are in a same tile, a same slice, orthe like. Referring to FIG. 13, the ratio N is 4, the search rangeincludes 3 previously reconstructed CTBs (i.e., the previouslyreconstructed CTBs (1321)-(1323) between the left most CTB (1310) andthe current CTB (1315)). The previously reconstructed CTBs (1321)-(1323)are fully available for reference, for example, if the previouslyreconstructed CTBs (1321)-(1323) and the current CTB (1315) are in asame tile or slice, and thus, no additional check for reference blockavailability is necessary.

In a third embodiment, the search range is extended to have the Npreviously reconstructed CTBs including the left most CTB, and specifichandling may be necessary. The current CTB and the left most CTB can bedivided into 4 regions of an equal size. In an example, the 4 regionsare square regions. Depending on which of the regions the current blockis located, a part of the left most CTB may or may not be available forreference, for example, similar to description with reference to FIGS.10, 11, and 12A-D.

In a first example of the third embodiment, the search range and a blockvector for the current block satisfy constraints that are suitablyadapted from the constraints described with reference to FIG. 10. Forexample, the modified constraints include the first conditions, thesecond conditions, modified third conditions, and modified fourthconditions. The modified third conditions can be specified as below:(yCb+(mvL[1]>>4))>>Ctb Log 2SizeY=yCb>>Ctb Log 2SizeY  (1)(yCb+(mvL[1]>>4+cbHeight−1)>>Ctb Log 2SizeY=yCb>>Ctb Log 2Size  (2)(xCb+(mvL[0]>>4))>>Ctb Log 2SizeY>=(xCb>>Ctb Log 2SizeY)−1<<((Max CtbLog 2SizeY−Ctb Log 2SizeY)<<1))  (5)(xCb+(mvL[0]>>4)+cbWidth−1)>>Ctb Log 2SizeY<=(xCb>>Ctb Log 2SizeY)  (4)where Eqs. (1)-(2) and (4) remain identical to those in the thirdconditions and Eq. (5) replaces Eq. (3) in the third conditions. Theparameter Max Ctb Log 2SizeY represents the maximum CTB size or thereference memory size in a log 2 form. As described above, the searchrange can include N previously reconstructed CTBs, such as the left mostCTB (1310) that is offset by N of the CTB width from the current CTB(1315) and (N−1) CTBs that are between the left most CTB (1310) and thecurrent CTB (1315) in the FIG. 13 example. Eqs. (4)-(5) constrain thereference block to be within one of: the left most CTB (1310), thecurrent CTB (1315), and the (N−1) CTBs (1321)-(1323).

The modified fourth conditions can specify that when the reference blockis in the left most CTB (1310), a collocated region for the referenceblock is not reconstructed (i.e., no samples in the collocated regionhave been reconstructed where the collocated region for the referenceblock is in the current CTB (1315)). The collocated region for thereference block is offset by N of the CTB width from a region where thereference block is located. For example, the modified fourth conditionscan be specified as below: when (xCb+mvL[0]>>4))>>Ctb Log 2SizeY isequal to (xCb>>Ctb Log 2SizeY)−1<<((Max Ctb Log 2SizeY−Ctb Log2SizeY)<<1)), the derivation process for reference block availability isinvoked with a position of a top left sample of the current block(xCurr, yCurr) set to be (xCb, yCb) and a position (((xCb+(mvL[0]>>4)+NCtbSizeY)>>(Ctb Log 2SizeY−1))<<(Ctb Log 2SizeY−1),((yCb+(mvL[1]>>4))>>(Ctb Log 2SizeY−1))<<(Ctb Log 2SizeY−1)) as inputs,an output is equal to FALSE indicating that the collocated region forthe reference block is not reconstructed.

In a second example of the third embodiment, the search range and theblock vector for the current block satisfy constraints that are suitablyadapted from the constraints described with reference to FIG. 10. Forexample, the modified constraints include the first conditions, thesecond conditions, modified third conditions, and modified fourthconditions. The modified third conditions can be identical to thatdescribed with reference to the first example of the third embodiment,and thus, detailed descriptions are omitted for purposes of brevity. Themodified fourth conditions include the modified fourth conditionsdescribed with reference to the first example of the third embodiment.Further, the modified fourth conditions include an additional conditionbelow: when Ctb Log 2SizeY is equal to Max Ctb Log 2SizeY, a position(((xCb+(mvL[0]>>4)+CtbSizeY)>>(Ctb Log 2SizeY−1))<<(Ctb Log 2SizeY−1),((yCb+mvL[1]>>4))>>(Ctb Log 2SizeY−1))<<(Ctb Log 2SizeY−1)) is not equalto (xCb, yCb). The additional condition ensures that a position of thecollocated region of the reference block is not equal to a position ofthe current block. In this regard, when the current block is to bereconstructed first in the current region, the reference block cannot bein a collocated region of the current region. Accordingly, the searchrange excludes the collocated region of the current region.

In a fourth embodiment, the left most CTB is set to be not available forreference, and thus, the search range excludes the left most CTB.Therefore, the search range can include a reconstructed part of thecurrent CTB and the (N−1) CTBs between the left most CTB and the currentCTB, similar to the second embodiment.

In a first example of the fourth embodiment, the search range and theblock vector for the current block satisfy constraints that are suitablyadapted from the constraints described with reference to FIG. 10. Forexample, the modified constraints include the first conditions, thesecond conditions, modified third conditions, and modified fourthconditions. The modified third conditions can be specified as below:(yCb+(mvL[1]>>4))>>Ctb Log 2SizeY=yCb>Ctb Log 2SizeY  (1)(yCb+(mvL[1]>>4+cbHeight−1)>>Ctb Log 2SizeY−yCb>>Ctb Log 2Size  (2)(xCb+(mvL[0]>>4))>>Ctb Log 2SizeY>=(xCb>>Ctb Log 2SizeY)−1<<((7−Ctb Log2SizeY)<<1))+Min(1,Max Ctb Log 2SizeY−Ctb Log 2SizeY)  (6)(xCb+(mvL[0]>>4)+cbWidth−1)>>Ctb Log 2SizeY<=(xCb>>Ctb Log 2SizeY)  (4)where Eqs. (1)-(2) and (4) remain identical to those in the thirdconditions and Eq. (6) replaces Eq. (3) in the third conditions. Eqs.(4) and (6) constrain the reference block to be within one of: thecurrent CTB and the (N−1) CTBs.

The modified fourth conditions can specify that when the reference blockis in the left neighbor of the current CTB and the CTB size is themaximum CTB size (also the reference memory size), a collocated regionfor the reference block is not reconstructed (i.e., no samples in thecollocated region have been reconstructed where the collocated regionfor the reference block is in the current CTB). For example, themodified fourth conditions can be specified as below: when(xCb+(mvL[0]>>4))>>Ctb Log 2SizeY is equal to (xCb>>Ctb Log 2SizeY)−1and Ctb Log 2SizeY is equal to Max Ctb Log 2SizeY, the derivationprocess for reference block availability is invoked with a position of atop left sample of the current block (xCurr, yCurr) set to be (xCb, yCb)and a position (((xCb+(mvL[0]>>4)+CtbSizeY)>>(Ctb Log 2SizeY−1))>>(CtbLog 2SizeY−1), ((yCb+(mvL[1]>>4))>>(Ctb Log 2SizeY−1))<<(Ctb Log2SizeY−1)) as inputs, an output is equal to FALSE indicating that thecollocated region for the reference block is not reconstructed.

In a second example of the fourth embodiment, the search range and theblock vector for the current block satisfy constraints that are suitablyadapted from the constraints described with reference to FIG. 10. Forexample, the modified constraints include the first conditions, thesecond conditions, modified third conditions, and modified fourthconditions. The modified third conditions can be identical to themodified third conditions of the first example of the fourth embodimentthat constrain the reference block to be within one of: the current CTBand the (N−1) CTBs.

The modified fourth conditions include the modified fourth conditions ofthe first example of the fourth embodiment. Therefore, when thereference block is in the left neighbor of the current CTB and the CTBsize is the maximum CTB size (also the reference memory size), acollocated region for the reference block is not reconstructed (i.e., nosamples in the collocated region have been reconstructed where thecollocated region for the reference block is in the current CTB).Further, the modified fourth conditions include an additional conditionas below: when Ctb Log 2SizeY is equal to Max Ctb Log 2SizeY, a position(((xCb+(mvL[0]>>4)+CtbSizeY)>>(Ctb Log 2SizeY−1))<<(Ctb Log 2SizeY−1),((yCb+(mvL[1]>>4))>>(Ctb Log 2SizeY−1))<<(Ctb Log 2SizeY−1)) is notequal to (xCb, yCb). The additional condition ensures that a position ofthe collocated region of the reference block is not equal to a positionof the current block. In this regard, when the current block is to bereconstructed first in the current region, the reference block cannot bein a collocated region of the current region. Accordingly, the searchrange excludes the collocated region of the current region.

In the above descriptions, a CTB can include 4 regions. For example, thecurrent CTB 1015 includes the regions (1016-1019). The descriptions canbe suitably adapted to scenarios where a CTB includes any suitablenumber of regions, and the number can be a positive integer. Inaddition, the regions can have any suitable size and shape includingrectangles, squares, or the like. In an example, a size of the regionscan be determined based on a reference memory size, a unit size formemory, and/or the like. In the examples described above, a region caninclude 9 coding blocks. In general, a region can include any suitablenumber of coding blocks, and the description can be suitably adapted.

FIG. 14 shows a flow chart outlining a process (1400) according to anembodiment of the disclosure. The process (1400) can be used in thereconstruction of a current block coded in intra block copy mode, so togenerate a reference block for the block under reconstruction. Invarious embodiments, the process (1400) are executed by processingcircuitry, such as the processing circuitry in the terminal devices(310), (320), (330) and (340), the processing circuitry that performsfunctions of the video encoder (403), the processing circuit thatperforms functions of the video decoder (410), the processing circuitrythat performs functions of the video decoder (510), the processingcircuitry that performs functions of the video encoder (603), and thelike. In some embodiments, the process (1400) is implemented in softwareinstructions, thus when the processing circuitry executes the softwareinstructions, the processing circuitry performs the process (1400). Theprocess starts at (S1401) and proceeds to (S1410).

At (S1410), prediction information of the current block is decoded froma coded video bitstream. The prediction information indicates the intrablock copy mode. The current block is one of a plurality of codingblocks in a current region of a current CTB in a current picture.

At (S1420), when the current block is to be reconstructed first in thecurrent region, a block vector is determined for the current block wherea reference block indicated by the block vector is in a search rangethat excludes a collocated region in a previously reconstructed CTB. Asdescribed above with reference to FIGS. 10, 11, 12A-12D, and 13, aposition of the collocated region in the previously reconstructed CTBhaving a same relative position as the current region in the currentCTB.

The search range is in the current picture. In an embodiment, the searchrange includes coding blocks that are reconstructed after the collocatedregion and before the current block.

In an embodiment, a CTB size can be compared with a reference memorysize. In an example, when the CTB size is equal to the reference memorysize, the previously reconstructed CTB is a left neighbor of the currentCTB, the position of the collocated region is offset by a width of thecurrent CTB from a position of the current region, and the coding blocksin the search range are in at least one of: the current CTB and thepreviously reconstructed CTB. In an example, the size of the current CTBand the previously reconstructed CTB is 128 by 128 samples, the currentCTB includes 4 regions of 64 by 64 samples, the previously reconstructedCTB includes 4 regions of 64 by 64 samples, the position of thecollocated region is offset by 128 samples from the position of thecurrent region, the current region being one of the 4 regions in thecurrent CTB and the collocated region being one of the 4 regions in thepreviously reconstructed CTB.

In an example, the CTB size is less than the reference memory size, anda ratio N between the reference memory size over the CTB size is largerthan 1. Accordingly, the position of the collocated region is offset byN of the CTB width from a position of the current region, and the codingblocks in the search range are in at least one of: the current CTB, theleft most previously reconstructed CTB that is left shifted by N of theCTB width from the current CTB, and (N−1) reconstructed CTBs between thecurrent CTB and the left most previously reconstructed CTB. For example,the CTB size is 64×64 samples, the reference memory size is 128×128samples, the current CTB includes 4 regions of 32×32 samples, thepreviously reconstructed CTB includes 4 regions of 32×32 samples, theposition of the collocated region is offset by 256 samples from theposition of the current region.

Alternatively, when the CTB size is less than the reference memory size,the search range excludes the left most previously reconstructed CTB. Inan example, the search range can include the (N−1) reconstructed CTBsand reconstructed part of the current CTB.

At (S1430), at least one sample of the current block is reconstructedaccording to the block vector. In an example, the reference block isobtained using the block vector, and the at least one sample is obtainedfrom the reference block. Then the process (1400) proceeds to (S1499)and terminates.

The process (1400) can be suitably adapted to various scenarios, forexample, when the current CTB includes a number of regions that isdifferent from 4 regions. In an embodiment, the process (1400) can alsobe used to reconstruct a coding block that is reconstructed afteranother coding block in the current region.

The techniques described above, can be implemented as computer softwareusing computer-readable instructions and physically stored in one ormore computer-readable media. For example, FIG. 15 shows a computersystem (1500) suitable for implementing certain embodiments of thedisclosed subject matter.

The computer software can be coded using any suitable machine code orcomputer language, that may be subject to assembly, compilation,linking, or like mechanisms to create code comprising instructions thatcan be executed directly, or through interpretation, micro-codeexecution, and the like, by one or more computer central processingunits (CPUs), Graphics Processing Units (GPUs), and the like.

The instructions can be executed on various types of computers orcomponents thereof, including, for example, personal computers, tabletcomputers, servers, smartphones, gaming devices, internet of thingsdevices, and the like.

The components shown in FIG. 15 for computer system (1500) are exemplaryin nature and are not intended to suggest any limitation as to the scopeof use or functionality of the computer software implementingembodiments of the present disclosure. Neither should the configurationof components be interpreted as having any dependency or requirementrelating to any one or combination of components illustrated in theexemplary embodiment of a computer system (1500).

Computer system (1500) may include certain human interface inputdevices. Such a human interface input device may be responsive to inputby one or more human users through, for example, tactile input (such as:keystrokes, swipes, data glove movements), audio input (such as: voice,clapping), visual input (such as: gestures), olfactory input (notdepicted). The human interface devices can also be used to capturecertain media not necessarily directly related to conscious input by ahuman, such as audio (such as: speech, music, ambient sound), images(such as: scanned images, photographic images obtain from a still imagecamera), video (such as two-dimensional video, three-dimensional videoincluding stereoscopic video).

Input human interface devices may include one or more of (only one ofeach depicted): keyboard (1501), mouse (1502), trackpad (1503), touchscreen (1510), data-glove (not shown), joystick (1505), microphone(1506), scanner (1507), camera (1508).

Computer system (1500) may also include certain human interface outputdevices. Such human interface output devices may be stimulating thesenses of one or more human users through, for example, tactile output,sound, light, and smell/taste. Such human interface output devices mayinclude tactile output devices (for example tactile feedback by thetouch-screen (1510), data-glove (not shown), or joystick (1505), butthere can also be tactile feedback devices that do not serve as inputdevices), audio output devices (such as: speakers (1509), headphones(not depicted)), visual output devices (such as screens (1510) toinclude CRT screens, LCD screens, plasma screens, OLED screens, eachwith or without touch-screen input capability, each with or withouttactile feedback capability some of which may be capable to output twodimensional visual output or more than three dimensional output throughmeans such as stereographic output; virtual-reality glasses (notdepicted), holographic displays and smoke tanks (not depicted)), andprinters (not depicted).

Computer system (1500) can also include human accessible storage devicesand their associated media such as optical media including CD/DVD ROM/RW(1520) with CD/DVD or the like media (1521), thumb-drive (1522),removable hard drive or solid state drive (1523), legacy magnetic mediasuch as tape and floppy disc (not depicted), specialized ROM/ASIC/PLDbased devices such as security dongles (not depicted), and the like.

Those skilled in the art should also understand that term “computerreadable media” as used in connection with the presently disclosedsubject matter does not encompass transmission media, carrier waves, orother transitory signals.

Computer system (1500) can also include an interface to one or morecommunication networks. Networks can for example be wireless, wireline,optical. Networks can further be local, wide-area, metropolitan,vehicular and industrial, real-time, delay-tolerant, and so on. Examplesof networks include local area networks such as Ethernet, wireless LANs,cellular networks to include GSM, 3G, 4G, 5G, LTE and the like, TVwireline or wireless wide area digital networks to include cable TV,satellite TV, and terrestrial broadcast TV, vehicular and industrial toinclude CANBus, and so forth. Certain networks commonly require externalnetwork interface adapters that attached to certain general purpose dataports or peripheral buses (1549) (such as, for example USB ports of thecomputer system (1500)); others are commonly integrated into the core ofthe computer system (1500) by attachment to a system bus as describedbelow (for example Ethernet interface into a PC computer system orcellular network interface into a smartphone computer system). Using anyof these networks, computer system (1500) can communicate with otherentities. Such communication can be uni-directional, receive only (forexample, broadcast TV), uni-directional send-only (for example CANbus tocertain CANbus devices), or bi-directional, for example to othercomputer systems using local or wide area digital networks. Certainprotocols and protocol stacks can be used on each of those networks andnetwork interfaces as described above.

Aforementioned human interface devices, human-accessible storagedevices, and network interfaces can be attached to a core (1540) of thecomputer system (1500).

The core (1540) can include one or more Central Processing Units (CPU)(1541), Graphics Processing Units (GPU) (1542), specialized programmableprocessing units in the form of Field Programmable Gate Areas (FPGA)(1543), hardware accelerators for certain tasks (1544), and so forth.These devices, along with Read-only memory (ROM) (1545), Random-accessmemory (1546), internal mass storage such as internal non-useraccessible hard drives, SSDs, and the like (1547), may be connectedthrough a system bus (1548). In some computer systems, the system bus(1548) can be accessible in the form of one or more physical plugs toenable extensions by additional CPUs, GPU, and the like. The peripheraldevices can be attached either directly to the core's system bus (1548),or through a peripheral bus (1549). Architectures for a peripheral businclude PCI, USB, and the like.

CPUs (1541), GPUs (1542), FPGAs (1543), and accelerators (1544) canexecute certain instructions that, in combination, can make up theaforementioned computer code. That computer code can be stored in ROM(1545) or RAM (1546). Transitional data can be also be stored in RAM(1546), whereas permanent data can be stored for example, in theinternal mass storage (1547). Fast storage and retrieve to any of thememory devices can be enabled through the use of cache memory, that canbe closely associated with one or more CPU (1541), GPU (1542), massstorage (1547), ROM (1545), RAM (1546), and the like.

The computer readable media can have computer code thereon forperforming various computer-implemented operations. The media andcomputer code can be those specially designed and constructed for thepurposes of the present disclosure, or they can be of the kind wellknown and available to those having skill in the computer software arts.

As an example and not by way of limitation, the computer system havingarchitecture (1500), and specifically the core (1540) can providefunctionality as a result of processor(s) (including CPUs, GPUs, FPGA,accelerators, and the like) executing software embodied in one or moretangible, computer-readable media. Such computer-readable media can bemedia associated with user-accessible mass storage as introduced above,as well as certain storage of the core (1540) that are of non-transitorynature, such as core-internal mass storage (1547) or ROM (1545). Thesoftware implementing various embodiments of the present disclosure canbe stored in such devices and executed by core (1540). Acomputer-readable medium can include one or more memory devices orchips, according to particular needs. The software can cause the core(1540) and specifically the processors therein (including CPU, GPU,FPGA, and the like) to execute particular processes or particular partsof particular processes described herein, including defining datastructures stored in RAM (1546) and modifying such data structuresaccording to the processes defined by the software. In addition or as analternative, the computer system can provide functionality as a resultof logic hardwired or otherwise embodied in a circuit (for example:accelerator (1544)), which can operate in place of or together withsoftware to execute particular processes or particular parts ofparticular processes described herein. Reference to software canencompass logic, and vice versa, where appropriate. Reference to acomputer-readable media can encompass a circuit (such as an integratedcircuit (IC)) storing software for execution, a circuit embodying logicfor execution, or both, where appropriate. The present disclosureencompasses any suitable combination of hardware and software.

APPENDIX A: ACRONYMS

-   JEM: joint exploration model-   VVC: versatile video coding-   BMS: benchmark set-   MV: Motion Vector-   HEVC: High Efficiency Video Coding-   SEI: Supplementary Enhancement Information-   VUI: Video Usability Information-   GOPs: Groups of Pictures-   TUs: Transform Units,-   PUs: Prediction Units-   CTUs: Coding Tree Units-   CTBs: Coding Tree Blocks-   PBs: Prediction Blocks-   HRD: Hypothetical Reference Decoder-   SNR: Signal Noise Ratio-   CPUs: Central Processing Units-   GPUs: Graphics Processing Units-   CRT: Cathode Ray Tube-   LCD: Liquid-Crystal Display-   OLED: Organic Light-Emitting Diode-   CD: Compact Disc-   DVD: Digital Video Disc-   ROM: Read-Only Memory-   RAM: Random Access Memory.-   ASIC: Application-Specific Integrated Circuit-   PLD: Programmable Logic Device-   LAN: Local Area Network-   GSM: Global System for Mobile communications-   LTE: Long-Term Evolution-   CANBus: Controller Area Network Bus-   USB: Universal Serial Bus-   PCI: Peripheral Component interconnect-   FPGA: Field Programmable Gate Areas-   SSD: solid-state drive-   IC: Integrated Circuit-   CU: Coding Unit-   SCC: Screen Content Coding-   VTM: Versatile test model-   BV: Block Vector-   AMVP: Advanced Motion Vector Prediction-   CPR: Current Picture Referencing-   IBC: Intra Block Copy-   DPB: Decoder Picture Buffer

While this disclosure has described several exemplary embodiments, thereare alterations, permutations, and various substitute equivalents, whichfall within the scope of the disclosure. It will thus be appreciatedthat those skilled in the art will be able to devise numerous systemsand methods which, although not explicitly shown or described herein,embody the principles of the disclosure and are thus within the spiritand scope thereof.

What is claimed is:
 1. A method for video decoding, comprising: decodingprediction information of a current block from a coded video bitstream,the prediction information being indicative of an intra block copy mode,the current block being one of a plurality of coding blocks in a currentregion, out of all regions, of a current coding tree block (CTB) in acurrent picture; determining whether the current block is to bereconstructed first in the current region; when the current block is tobe reconstructed first in the current region, determining a block vectorfor the current block, a reference block indicated by the block vectorbeing in a search range that excludes a collocated region in apreviously reconstructed CTB, a position of the collocated region in thepreviously reconstructed CTB having a same relative position as thecurrent region in the current CTB, the search range being in the currentpicture; and reconstructing at least one sample of the current blockaccording to the block vector, wherein the search range includes codingblocks that are reconstructed before the current block in the currentCTB or coding blocks that are not excluded in the previouslyreconstructed CTB, wherein a size of the current CTB is less than orequal to a reference memory size, the previously reconstructed CTB is aleft immediately adjacent neighbor of the current CTB, the position ofthe collocated region is offset by a width of the current CTB from aposition of the current region, and the coding blocks in the searchrange include at least all remaining regions of the previouslyreconstructed CTB that have not been excluded from the search range whenthe current region is not a final region to be reconstructed in thecurrent CTB, and wherein the method includes sequentially reconstructingeach of the regions of the current CTB by updating the search range tonewly exclude a collocated region in the previously reconstructed CTBwhen the current region changes.
 2. The method of claim 1, wherein thesize of the current CTB and the previously reconstructed CTB is 128 by128 samples, the current CTB includes 4 regions of 64 by 64 samples, thepreviously reconstructed CTB includes 4 regions of 64 by 64 samples, theposition of the collocated region is offset by 128 samples from theposition of the current region, the current region being one of the 4regions in the current CTB and the collocated region being one of the 4regions in the previously reconstructed CTB.
 3. The method of claim 2,wherein the 4 regions in the current CTB includes a top left region, atop right region, a bottom left region, and a bottom right region; the 4regions in the previously reconstructed CTB includes a top left region,a top right region, a bottom left region, and a bottom right region; andthe sequentially reconstructing each of the regions includes performingall of the following: when the current region is the top left region ofthe current CTB, the collocated region is the top left region of thepreviously reconstructed CTB and the search region is updated to excludethe top left region of the previously reconstructed CTB; when thecurrent region is the top right region of the current CTB, thecollocated region is the top right region of the previouslyreconstructed CTB and the search region is updated to exclude the topleft region and the top right region of the previously reconstructedCTB; when the current region is the bottom left region of the currentCTB, the collocated region is the bottom left region of the previouslyreconstructed CTB and the search region is updated to exclude the topleft region, the top right region, and the bottom left region of thepreviously reconstructed CTB; and when the current region is the bottomright region of the current CTB, the collocated region is the bottomright region of the previously reconstructed CTB and the search regionis updated to exclude the previously reconstructed CTB.
 4. The method ofclaim 1, wherein the current CTB includes 4 regions having a same sizeand shape, the previously reconstructed CTB includes 4 regions havingthe same size and the shape, the current region is one of the 4 regionsin the current CTB, and the collocated region is one of the 4 regions inthe previously reconstructed CTB.
 5. The method of claim 1, wherein asize of the current CTB is less than a reference memory size, theposition of the collocated region is offset by multiple widths of thecurrent CTB from a position of the current region, and the coding blocksin the search range are in at least one of: the current CTB, thepreviously reconstructed CTB, and one or more reconstructed CTBs betweenthe current CTB and the previously reconstructed CTB.
 6. The method ofclaim 5, wherein the size of the current CTB is 64×64 samples, thereference memory size is 128×128 samples, the current CTB includes 4regions of 32×32 samples, the previously reconstructed CTB includes 4regions of 32×32 samples, the position of the collocated region isoffset by 256 samples from the position of the current region.
 7. Themethod of claim 5, wherein the search range excludes the previouslyreconstructed CTB that is offset by N widths of the current CTB from thecurrent CTB and N is a ratio of the reference memory size over the sizeof the current CTB.
 8. The method of claim 1, wherein the coding blocksin the search range include only regions in the current CTB that arereconstructed immediately after the collocated region when the currentregion is the final region to be reconstructed in the current CTB.
 9. Anapparatus for video decoding, comprising: processing circuitryconfigured to: decode prediction information of a current block from acoded video bitstream, the prediction information being indicative of anintra block copy mode, the current block being one of a plurality ofcoding blocks in a current region, out of all regions, of a currentcoding tree block (CTB) in a current picture; determine whether thecurrent block is to be reconstructed first in the current region; whenthe current block is to be reconstructed first in the current region,determine a block vector for the current block, a reference blockindicated by the block vector being in a search range that excludes acollocated region in a previously reconstructed CTB, a position of thecollocated region in the previously reconstructed CTB having a samerelative position as the current region in the current CTB, the searchrange being in the current picture; and reconstruct at least one sampleof the current block according to the block vector, wherein the searchrange includes coding blocks that are reconstructed before the currentblock in the current CTB or coding blocks that are not excluded in thepreviously reconstructed CTB, wherein a size of the current CTB is lessthan or equal to a reference memory size, the previously reconstructedCTB is a left immediately adjacent neighbor of the current CTB, theposition of the collocated region is offset by a width of the currentCTB from a position of the current region, and the coding blocks in thesearch range include at least all remaining regions of the previouslyreconstructed CTB that have not been excluded from the search range whenthe current region is not a final region to be reconstructed in thecurrent CTB, and wherein the processing circuitry is configured tosequentially reconstruct each of the regions of the current CTB byupdating the search range to newly exclude a collocated region in thepreviously reconstructed CTB when the current region changes.
 10. Theapparatus of claim 9, wherein the size of the current CTB and thepreviously reconstructed CTB is 128 by 128 samples, the current CTBincludes 4 regions of 64 by 64 samples, the previously reconstructed CTBincludes 4 regions of 64 by 64 samples, the position of the collocatedregion is offset by 128 samples from the position of the current region,the current region being one of the 4 regions in the current CTB and thecollocated region being one of the 4 regions in the previouslyreconstructed CTB.
 11. The apparatus of claim 9, wherein the current CTBincludes 4 regions having a same size and shape, the previouslyreconstructed CTB includes 4 regions having the same size and the shape,the current region is one of the 4 regions in the current CTB, and thecollocated region is one of the 4 regions in the previouslyreconstructed CTB.
 12. The apparatus of claim 9, wherein a size of thecurrent CTB is less than a reference memory size, the position of thecollocated region is offset by multiple widths of the current CTB from aposition of the current region, and the coding blocks in the searchrange are in at least one of: the current CTB, the previouslyreconstructed CTB, and one or more reconstructed CTBs between thecurrent CTB and the previously reconstructed CTB.
 13. The apparatus ofclaim 12, wherein the size of the current CTB is 64×64 samples, thereference memory size is 128×128 samples, the current CTB includes 4regions of 32×32 samples, the previously reconstructed CTB includes 4regions of 32×32 samples, the position of the collocated region isoffset by 256 samples from the position of the current region.
 14. Theapparatus of claim 12, wherein the search range excludes the previouslyreconstructed CTB that is offset by N widths of the current CTB from thecurrent CTB and N is a ratio of the reference memory size over the sizeof the current CTB.
 15. A non-transitory computer-readable mediumstoring instructions which when executed by a computer for videodecoding cause the computer to perform a method comprising: decodingprediction information of a current block from a coded video bitstream,the prediction information being indicative of an intra block copy mode,the current block being one of a plurality of coding blocks in a currentregion, out of all regions, of a current coding tree block (CTB) in acurrent picture; determining whether the current block is to bereconstructed first in the current region; when the current block is tobe reconstructed first in the current region, determining a block vectorfor the current block, a reference block indicated by the block vectorbeing in a search range that excludes a collocated region in apreviously reconstructed CTB, a position of the collocated region in thepreviously reconstructed CTB having a same relative position as thecurrent region in the current CTB, the search range being in the currentpicture; and reconstructing at least one sample of the current blockaccording to the block vector, wherein the search range includes codingblocks that are reconstructed before the current block in the currentCTB or coding blocks that are not excluded in the previouslyreconstructed CTB, wherein a size of the current CTB is less than orequal to a reference memory size, the previously reconstructed CTB is aleft immediately adjacent neighbor of the current CTB, the position ofthe collocated region is offset by a width of the current CTB from aposition of the current region, and the coding blocks in the searchrange include at least all remaining regions of the previouslyreconstructed CTB that have not been excluded from the search range whenthe current region is not a final region to be reconstructed in thecurrent CTB, and wherein the method includes sequentially reconstructingeach of the regions of the current CTB by updating the search range tonewly exclude a collocated region in the previously reconstructed CTBwhen the current region changes.